*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-1

Chapter 3: Test Procedures

Test procedures are a crucial aspect of the heavy-duty vehicle GHG and fuel 

consumption program. This rulemaking is proposing to establish several new test procedures for 

both engine and vehicle compliance. This chapter will describe the development process for the 

test procedures being proposed, including the assessment of engines, aerodynamics, rolling 

resistance, chassis dynamometer testing, powertrain testing, and duty cycles.

3.1 Heavy-Duty Engine Test Procedure 

The agencies are controlling heavy-duty engine fuel consumption and greenhouse gas 

emissions through the use of engine certification. The program will mirror existing engine 

regulations for the control of both GHG and non-GHG pollutants in many aspects. The 

following sections provide an overview of the test procedures.

3.1.1 Existing Regulation Reference 

Heavy-duty engines currently are certified for GHG and non-GHG pollutants using test 

procedures developed by EPA. The Heavy-Duty Federal Test Procedure (FTP) is a transient test 

consisting of second-by-second sequences of engine speed and torque pairs with values given in 

normalized percent of maximum form. The cycle was computer generated from a dataset of 88 

heavy-duty trucks in urban operation in New York and Los Angeles. These procedures are welldefined, mirror in-use operating parameters, and thus we believe appropriate also for the 

assessment of GHG emissions from heavy duty engines. Further, EPA is concerned that we 

maintain a regulatory relationship between the non-GHG emissions and GHG emissions, 

especially for control of CO2 and NOX. Therefore, the agencies are proposing to continue using

the same criteria pollutant test procedures for both the CO2 and fuel consumption standards.

For 2007 and later Heavy-Duty engines, 40 CFR Parts 86 – “Control of Emissions from 

New and In-Use Highway Vehicles and Engines” and 1065 – “Engine Testing Procedures” detail 

the certification process. 40 CFR 86.007-11 defines the standard settings of Oxides of Nitrogen, 

Non-Methane Hydrocarbons, Carbon Monoxide, and Particulate Matter. The duty cycles are 

defined in Part 86. The Federal Test Procedure engine test cycle is defined in 40 CFR part 86 

Appendix I. The Supplemental Emissions Test engine cycle is defined in 40 CFR 86.1360(b). 

All emission measurements and calculations are defined in Part 1065, with exceptions as noted 

in 40 CFR 86.007-11. The data requirements are defined in 40 CFR 86.001-23 and 40 CFR 

1065.695.

The measurement method for CO2 is described in 40 CFR 1065.250. For measurement of 

CH4 refer to 40 CFR 1065.260. For measurement of N2O refer to 40 CFR 1065.275. We 

recommend that you use an analyzer that meets performance specifications shown in Table 1 of 

40 CFR 1065.205. Note that your system must meet the linearity verification of 40 CFR 

1065.307. To calculate the brake specific mass emissions for CO2, CH4 and N2O refer to 40 

CFR 1065.650. 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-2

3.1.2 Engine Dynamometer Test Procedure Proposed Modifications 

3.1.2.1 Fuel Consumption Calculation

EPA and NHTSA propose to calculate fuel consumption, as defined as gallons per brake 

horsepower-hour, from the CO2 measurement, just as in the HD Phase 1 rule. The agencies are 

proposing that manufacturers use 8,887 grams of CO2 per gallon of gasoline and 10,180 g CO2

per gallon of diesel fuel. 

3.1.2.2 Regeneration Impact on Fuel Consumption and CO2 Emissions 

The current engine test procedures also require the development of regeneration emission 

rate and frequency factors to account for the emission changes during a regeneration event.1 In

Phase 1, the agencies adopted provisions to exclude CO2 emissions and fuel consumption due to 

regeneration. However, for Phase 2, we propose to include CO2 emissions and fuel consumption 

due to regeneration over the FTP and RMC cycles as determined using the infrequently 

regenerating aftertreatment devices (IRAF) provisions in 40 CFR 1065.680. However, we are 

not proposing to include fuel consumption due to regeneration in the creation of the fuel map 

used in GEM for vehicle compliance Our assessment of the current non-GHG regulatory

program indicates that engine manufacturers have significantly reduced the frequency of 

regeneration events. In addition, market forces already exist which create incentives to reduce 

fuel consumption during regeneration. 

3.1.2.3 Fuel Heating Value Correction

In the HD Phase 1 rule, the agencies collected baseline CO2 performance of diesel 

engines from testing which used fuels with similar properties. The agencies are proposing to 

continue using a fuel-specific correction factor for the fuel’s energy content in case this changes 

in the future. The agencies found the average energy content of the diesel fuel used at EPA’s 

National Vehicle Fuel and Emissions Laboratory was 21,200 BTU per pound of carbon. This 

value is determined by dividing the Net Heating Value (BTU per pound) by the carbon weight 

fraction of the fuel used in testing. We are also proposing to continue using the Phase 1 

corrections for gasoline, natural gas, and liquid petroleum gas in 40 CFR 1036.530. We are also 

proposing to expand the table by adding dimethyl ether.

In addition to the fuel heating value correction, we are proposing the addition of 

reference carbon mass fraction values for these fuels to the Table in 40 CFR 1036.530. These 

reference values are used in the powertrain calculations 40 CFR 1037.550 to account for the 

difference in carbon mass fraction between the test fuel and the reference fuel prior to correcting 

for the test fuel’s mass-specific net energy content. 

The agencies are not proposing fuel corrections for alcohols because the fuel chemistry is

homogeneous.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-3

3.1.2.4 Urea Derived CO2 Correction

The agencies are proposing to allow manufacturers to correct compression ignition 

engine and powertrain CO2 emission results (for engines utilizing urea SCR for NOX control) to 

account for the contribution of urea derived CO2 emissions to the total engine CO2 emissions.

Urea derived CO2 can account for up to 1 percent of the total CO2 emissions. Urea is 

produced from gaseous NH3 and gaseous CO2 that is captured from the atmosphere, thus CO2

derived from urea decomposition in diesel SCR emission control systems results in a net 

emission of zero CO2 to the environment. In our proposed test procedures for Phase 2, we allow 

manufacturers to determine CO2 emissions either by measuring the CO2 emitted from the engine 

or to determine it by measuring fuel flow rate during the test. If we do not allow for correction 

of the urea derived CO2 emissions, this will result in a positive CO2 bias for CO2 emissions 

determined by measuring the CO2 emitted from the engine. To perform this correction, we are 

proposing that you determine the mass rate of urea injected over the duty cycle from the engine’s 

J1939 CAN signal. This value is used as an input to an equation that allows you to determine the 

mass rate of CO2 from urea during the duty cycle. This resulting CO2 mass emission rate value 

is then used as an input to the steady-state fuel map fuel mass flow rate calculation in 40 CFR 

1036.535 and the total mass of CO2 emissions over the duty cycle calculations in 40 CFR 

1037.550. Note that this correction is only allowed for CO2 measured from the engine and not 

CO2 derived from fuel flow measurement.

The calculation for determination of the mass rate of CO2 from urea requires the user to 

input the urea solution urea percent by mass. This calculation uses prescribed molecular weights 

for CO2 and urea as given in 40 CFR 1065.1005 of 44.0095 and 60.05526 respectively. A 1:1 

molar ratio of urea reactant to CO2 product is assumed.

To facilitate the ability of the agencies to make this correction, we are proposing that the 

urea mass flow rate be broadcasted on the non-proprietary J1939 PGN (Parameter Group 

Number) 61475 (and 61478 if applicable).

3.1.2.5 Multiple Fuel Maps 

Modern heavy-duty engines may have multiple fuel maps, commonly meant to improve 

performance or fuel efficiency under certain operating conditions. CO2 emissions can also be 

different depending on which map is tested, so it is important to specify a procedure to properly

deal with engines with multiple fuel maps. Consistent with criteria-pollutant emissions 

certification, engine manufacturers should submit CO2 data from all fuel maps on a given test 

engine. This includes fuel map information as well as the conditions under which a given fuel 

map is used (i.e. transmission gear, vehicle speed, etc).

3.1.2.6 Measuring GEM Engine Inputs

To recognize the contribution of the engine in GEM the engine fuel map, full load torque 

curve and motoring torque curve have to be input into GEM. To insure the robustness of each of 

those inputs, a standard procedure has to be followed. Both the full load and motoring torque 

curve procedures are already defined in 40CFR part 1065 for engine testing. However, the fuel 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-4

mapping procedure being proposed would be new. The agencies have compared the proposed 

procedure against other accepted engine mapping procedures with a number of engines at 

various labs including EPA’s NVFEL, Southwest Research Institute sponsored by the agencies, 

and Environment Canada’s laboratory.

1

 The proposed procedure was selected because it proved 

to be accurate and repeatable, while limiting the test burden to create the fuel map. This proposed 

provision is consistent with NAS’s recommendation (3.8).

The agencies are proposing that engine manufacturers must certify fuel maps as part of 

their certification to the engine standards, and that they be required to provide those maps to 

vehicle manufacturers. The one exception to this requirement would be for cases in which the 

engine manufacturer certifies based on powertrain testing, as described in Section 3.6. In such 

cases, engine manufacturers would not be required to also certify the otherwise applicable fuel 

maps. We are not proposing that vehicle manufacturers will be allowed to develop their own 

fuel maps for engines they do not manufacture.

3.1.3 Engine Family Definition and Test Engine Selection

3.1.3.1 Criteria for Engine Families

The current regulations outline the criteria for grouping engine models into engine 

families sharing similar emission characteristics. A few of these defining criteria include borecenter dimensions, cylinder block configuration, valve configuration, and combustion cycle; a 

comprehensive list can be found in 40 CFR 86.096-24(a)(2). While this set of criteria was 

developed with criteria pollutant emissions in mind, similar effects on CO2 emissions can be 

expected. For this reason, this methodology should continue to be followed when considering 

CO2 emissions, just as it was in the HD Phase 1 rule.

3.1.3.2 Emissions Test Engine

We are proposing that manufacturers select at least one engine per engine family for 

emission testing. The methodology for selecting the test engine(s) should be consistent with 40 

CFR 86.096-24(b)(2) (for heavy-duty Otto cycle engines) and 40 CFR 86.096-24(b)(3) (for 

heavy-duty diesel engines). An inherent characteristic of these methodologies is selecting the 

engine with the highest fuel feed per stroke (primarily at the speed of maximum rated torque and 

secondarily at rated speed) as the test engine, as this is expected to produce the worst-case 

criteria pollutant emissions. To be consistent, however, it is recommended that the same 

methodology continue to be used for selecting test engines. 

3.2 Aerodynamic Assessment

3.2.1 Aerodynamics for Tractors

For the Phase 1 rule, the agencies promulgated requirements whereby the coefficient of 

drag assessment was a product of test data and modeling using good engineering judgment. A 

 

1 U.S. Environmental Protection Agency. Memo to Docket EPA-HQ-OAR-2014-0827.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-5

group of aerodynamic bins for tractors corresponding to certain known aerodynamic design 

features (e.g., Classic, Conventional, SmartWay, etc.) were established based on the results of an 

agency sponsored aerodynamic assessment test program. The rules require tractor manufacturers 

to take the aerodynamic test result from a tractor and determine the tractor’s appropriate bin. To 

ensure the consistency of the drag assessment results, certain aspects of the truck were defined, 

including the trailer, location of payload, and tractor-trailer gap. In addition, the agencies 

specified test procedures for aerodynamic assessment: coastdown testing (also used as the 

reference method), wind tunnel testing (reduced and full scale), and computational fluid 

dynamics (CFD). Constant speed testing was also permitted as an alternative test procedure, but 

the agencies did not develop a specific procedure.

For the HD Phase 2 proposal, we are retaining all the current HD Phase 1 aspects of the 

aerodynamic assessment protocols with the following revisions and additions: enhancement of 

the analysis methodology for the coastdown test procedure, which we are proposing to keep as 

the reference method for the proposed tractor program; specifications for the constant speed test 

procedure; inclusion of trailers in the aerodynamic assessment test protocols; modifications to 

the reference trailer used for tractor aerodynamic assessment and establishing a reference tractor 

for trailer aerodynamic assessment; proposal of wind-average coefficient of drag (CDwa) as the 

required aerodynamic Greenhouse Gas Emissions Model (GEM) input for tractors; and 

considering potential aerodynamic performance for advanced aerodynamic performance of 

tractor-trailer combinations in the proposed timeframe for this rulemaking. Another proposed 

modification to the aerodynamic assessment for HD Phase 2 is the use of drag area (coefficient 

of drag multiplied by the frontal area, or CdA), rather than the coefficient of drag (Cd), for tractor 

aerodynamic bin standards. Although this modification would not alter the aerodynamic 

assessment protocols, it is important to note this since all HD Phase 2 aerodynamic assessment 

results will be presented in this format, rather than the Cd format used for HD Phase 1.

3.2.2 Modifications to Aerodynamic Assessment Methods for HD Phase 2

Currently, tractor manufacturers are successfully using the established aerodynamic 

assessment methods established under HD Phase 1 for implementation and compliance with HD

Phase 1 emissions standards. Accordingly for HD Phase 2, we are proposing to continue to use 

the existing aerodynamic assessment methods for generating aerodynamic inputs to GEM with 

the coastdown test procedure as the reference method, and the constant speed test procedure, 

wind tunnels, and CFD as the allowed alternative methods (or any other EPA pre-approved test 

methods). As a result, for HD Phase 2, we are only considering modifications to further 

enhance, improve or specify the existing aerodynamic assessment methods. 

During development and since the beginning of HD Phase 1 implementation, we have 

received suggestions for improving the coastdown test procedure analysis methodology to reduce 

data post processing and improve data resolution. Also, as mentioned above, although constant 

speed testing is allowed as an alternative method, we did not define the specifications and 

protocols for conducting the testing as was done for the coastdown test procedure reference 

method and other alternative methods. Finally, by virtue of CFD being software-based, it may

be possible to improve the conditions specified for performing CFD analysis to provide a more 

realistic result. Accordingly, for HD Phase 2 aerodynamic assessment methods, we are 

proposing to modify the coastdown test procedure analysis methodology, define the 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-6

specifications and protocols for conducting and analyzing the results of the constant speed test 

procedure, and update the conditions for performing CFD analysis.

For HD Phase 2, we are not proposing any changes to the existing wind tunnel 

specifications and protocols other than revising the measured yaw angles to incorporate the wind 

average coefficient of drag. Under HD Phase 1, we only required manufacturers to conduct wind 

tunnel testing at a zero degree yaw angle. In contrast, for HD Phase 2, we are proposing to 

incorporate the wind average coefficient of drag and, consequently, to require measurement at 

additional yaw angles for generating the yaw sweep curve and calculating the wind average 

coefficient of drag.

Wind tunnels are pivotal in redefining a standard trailer for tractors, assessing different 

trailer types to support the proposed trailer standards, and assessing wind averaged drag. 

Therefore, the test results using the existing wind tunnel specifications and protocols, and the 

revisions to incorporate the wind average coefficient of drag will be discussed in the context of 

these areas later in this section.

3.2.2.1 Modification of Coastdown Testing Data Analysis Procedures for HD

Phase 2

Based on feedback from the heavy-duty vehicle manufacturing industry and other 

entities, the agencies finalized a Modified SAE J1263 coastdown procedure in the HD Phase 1 

rulemaking. During and since the finalization of HD Phase 1 regulations, stakeholders have 

suggested analyzing portions of the data rather than the full data set generated during coastdown 

testing to increase measurement accuracy and/or precision. One OEM suggested the use of the 

high speed portion of the coastdown test procedure speed range to solely or predominantly 

isolate the aerodynamic forces. Another OEM suggested using the high speed and low speed 

portions of the coastdown test procedure speed range in an iterative fashion to isolate the 

mechanical/frictional losses and rolling resistance predominantly present at lower speeds and 

removing these forces from the higher speed forces to capture predominantly aerodynamic 

forces.

As a result of these suggestions, the agencies (via contractors ICF Corporation and 

Southwest Research Institute (SwRI)) coasted down combination tractors on Farm-to-Market 

Highway 70, a rural highway, between Bishop, Texas and Chapman Ranch, Texas. A grade 

survey was performed by SwRI. Filtered USGS elevation data were also obtained for the same 

stretch of roadway.

2

The grade information was incorporated into our analysis. The testing was 

conducted overnight, usually between 12 am and 4 am, to minimize traffic and wind. To get a 

comprehensive data set to conduct various analysis techniques, the vehicles were coasted down 

from 70 mph to 0 mph. Approximately 20 runs (10 in each direction) were planned for each test, 

but the number was reduced to 14 runs (7 in each direction), due to the increase in test time 

associated with coasting all the way to 0 mph. An ultrasonic anemometer was mounted 0.85 m 

above the leading edge of the trailer at the midpoint of the trailer width. This anemometer 

recorded air speed and direction onboard the vehicle at 10 Hz. A weather station, which 

measured wind speed, wind direction, temperature, and air pressure at 1 Hz, was placed 

alongside the road at the approximate midpoint of the stretch of road being used for the tests. 

Details of the test setup and vehicle information can be found in the on-road testing summary
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-7

report from SwRI.3 The average and maximum wind speeds were calculated for each run to 

determine validity of the run with respect the wind restrictions. Table 3-1 below shows the

ambient conditions desired during coastdown testing, which resembles the SAE J1263 

recommended practice. 

Table 3-1 Coastdown Ambient Conditions

PARAMETER RANGE

Average wind speed at the test site

(for each run in each direction)

< 10 mph

Maximum wind speed (for each run in each 

direction)

<12.3 mph

Average cross wind speed

(for each run in each direction at the site)

< 5 mph

The position of the onboard anemometer is such that the air speed readings need to be 

corrected. Located above the trailer, the anemometer’s air velocity readings will typically be 

greater than the free stream air speed. The roadside weather station was used to correct the 

onboard air speed measurements, using the trigonometric calculations below. For this 

correction, each coastdown run was split into 5-mph segments, over which the vehicle speed v, 

measured air speed vr,meas, wind speed w, and wind direction θw were averaged.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-8

Figure 3-1 Diagram of vehicle speed and air speed vectors during coastdowns in opposite directions

v

r, th

w

θ

r, th

v

r, th

w

r, th

Direction 1

Direction 2
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-9

The law of cosines was used to calculate the theoretical air speed vr,th from the vehicle 

speed and weather station measurements, as described in the equation below. The bars over the 

variables indicate averages over the 5-mph segments.

r,th =

⎩

⎨

⎧

2 + 

2 − 2 cos 

w ,  = 1



2 + 

2 + 2 cos 

w ,  = 2

The resulting theoretical air speed values were regressed against the measured air speed 

values for every test, and this linear relationship was used to correct the air speed measurements 

in the real-time data. This relationship is shown by the equations below and from the test results 

given in Figure 3-2.

Regression equation for air speed correction: r,th = 0 + 1r,meas

Applied to air speed measurements:  =  + ,

Figure 3-2 Example of theoretical air speed vs. measured air speed shows a consistent relationship that can 

be used to correct the onboard air speed measurements.

vr,th = -0.872 + 0.937vr,meas [mph]
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-10

The 10-Hz data were filtered using a one-second weighted centered moving average prior 

to further analysis.

3.2.2.1.1 High-low Iteration Analysis

This analysis involves analyzing the coastdown over two separate speed ranges. A lowspeed range is used to estimate mechanical losses and subtract them out of a high-speed range to 

estimate aerodynamic drag. This process is iterated until mechanical and aerodynamic drag 

forces converge. The force is not calculated at each measurement, but instead the net force over 

each speed range is calculated by measuring the time taken to decelerate through each speed 

range. We assumed a linear decrease in speed (i.e. constant deceleration), because the speed 

ranges are small. We are also incorporating a simple speed-dependent rear axle loss adjustment 

to subtract out what we have learned to be a small but speed-dependent non-aerodynamic drag. 

We are also in the process of collecting data on the speed effect of tire rolling resistance and are 

considering including a simple adjustment for this in the final rule.

While this analysis can be done for any pair of speed ranges, we are focusing this 

discussion on a low speed range of 25 to 15 mph and a high speed range of 70 to 60 mph. Table 

3-2 below describes the analysis methodology step by step.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-11

Table 3-2 Drag Area Calculation Steps for High-Low Iteration Analysis

STEP 0: FIND THE TIMES 

BRACKETING THE LOWSPEED AND HIGH-SPEED 

RANGES

VLO1<VI<VLO2 (LOW SPEED)

VHI1<VI<VHI2 (HIGH SPEED)

,,, V = VEHICLE SPEED

LO1=15MPH, 

LO2=25MPH

HI1=60MPH, 

HI2=70MPH

Step 1: Calculate acceleration 

for each speed range.  =

 − 

 − 

=

∆

∆

 =

 − 

 − 

=

∆

∆

a = vehicle acceleration

t = time

Step 2: Calculate average road 

grade force over each speed 

range.

, =  

∆h

∆





= 

h − h

 − 

, =  

∆h

∆





= 

h − h

 − 

M = vehicle mass

h = elevation (relative)

s = travel distance

g = gravitational 

acceleration = 9.81 m/s2

Step 3: Inertial and Effective 

Mass

(Add 125 lbm per tire to 

account for rotational inertia).

 = 125





∙ 

= 56.7





∙ 

 =  + 

Minertial = additional inertia 

from rotating components

Me = effective mass

ntires = total number of tires 

in test configuration

Step 4: Road load force for 

each speed range, also 

accounting for rear axle loss 

estimate (Faxle).

 = − + , − ,

 = − + , − ,

F = force

Faxle,lo = 100 N

Faxle,hi = 200 N

Step 5: Air density during 

each high speed section.  =

1000 × P

R(T + 273.15)

ρ = density of air

P= average ambient 

pressure during high speed 

run in kPa

T = average ambient 

temperature during high 

speed run in °C

R = gas constant for air 

=287.058 J/(kg-K)

Step 6: Average relative air 

speed over each speed range.

,, = ∑







 ,, = ∑









vr = relative air speed

Step 7: Initial conditions (i=0). 

Start with no aerodynamic 

forces in the low speed range.

,, = 0
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-12

Step 8: Subtract low-speed 

aerodynamic forces from low

speed forces to estimate 

mechanical forces. 

, =  − ,,

Step 9: Subtract mechanical 

forces from high speed forces 

to estimate aerodynamic 

forces.

,, =  − ,

Step 10: Adjust aerodynamic 

forces by speed to estimate 

low-speed aerodynamic 

forces.

,, = ,, 

,,



,,





Step 11: Repeat steps 8-10 

until both high-speed 

aerodynamic and low-speed 

mechanical forces both 

converge less than 1%.

Repeat steps 8-10 until:

1 −

,,

,,

 < 0.01

and

1 −

,,

,,

 < 0.01

Step 12: Calculate drag area.

 =

2,,

,,



There are some advantages to using this method over the Phase 1 method. Focusing on 

segmented speed ranges may open up more test locations, as less road or track space would be 

required to collect a full data set. The middle range of speeds that would be eliminated contains 

a higher proportion of rolling resistance forces and also sweeps through greater yaw angles, even 

at modest crosswind conditions, which can increase the aerodynamic drag of certain runs and 

subsequently increase the variability of a test. This method does not account for yaw angle, but 

with the proper wind constraints and the use of a high-speed range to estimate aerodynamic drag, 

the yaw angle effect should be small or statistically indiscernible. The result of this analysis 

method is considered to be a zero-yaw CdA.

3.2.2.1.2 High-low Intercept Analysis

This method is similar to the previous method, with a few important differences. Instead 

of calculating force over speed intervals, like in the iteration method, the force is calculated at

every speed measurement, similar to the HD Phase 1. The mechanical forces are determined 

through the low speed range that goes all the way to 0 mph though a force versus vehicle speed 

regression. The force intercept is then adjusted by a generic rolling resistance speed adjustment

to estimate rolling resistance forces throughout the coastdown. These forces are then subtracted 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-13

from the total road load forces to estimate aerodynamic drag forces. The aerodynamic drag 

forces are then adjusted for yaw angle through an adjustment previous determined by a CFD run. 

The agencies did not evaluate this method in detail for the proposal due to lack of CFD or 

other yaw sweep data for the specific tractors that were tested. The agencies also did not have 

data to support a specific rolling resistance speed correction. The agencies are in the process of 

collecting some of this data and may evaluate this method for the final rule.

3.2.2.1.3 High-speed Yaw Angle Analysis

Another suggested analysis method was to use only the high-speed range from each 

coastdown (70-50 mph) and use the onboard anemometry to develop a wind-average CdA, as 

opposed to a zero-yaw CdA. One main advantage to this method is that it would allow for less 

road or track space to be used because only a high speed range would be needed. This allows for 

minimal acceleration and deceleration, which consume significant amounts of space and time, 

and subsequently allows for more runs to be conducted.

A second-order regression with no linear term is used to separate the mechanical forces 

from the aerodynamic forces; the regression is done similar to Phase 1, but using the smaller 

speed range and air speed instead of vehicle speed. 

 = 

∆

∆

=  + 



The D coefficient is used to estimate drag area CdA, using the temperature and pressure 

during the speed range to calculate air density, just as in the analysis methods discussed earlier in 

this section. The yaw angle θr is averaged over the speed range, and the absolute value is used. 

The CdA is fit to a second order regression with the absolute value of the yaw angle.

 =

2





d

 = 0 + 1r + 

2

r



2

This CdA curve would then be used to conduct a yaw sweep to develop a wind-average 

CdA value for certification, rather than a zero-yaw CdA. While air direction was measured 

onboard, the accuracy of the anemometer is only ±2°, according the product specifications.

4

Over the speed range recommend for analysis, yaw angles would only occur between 0° (for a 

direct headwind/tailwind) and 6° (for a direct crosswind). The accuracy of the instrument is not 

sufficient to measure average yaw angle. Instead, the roadside weather station was used to more 

accurately determine the average yaw angle. Trigonometric equations using the weather station 

measurements were used to calculate the average yaw angle for each coastdown run; the 

anemometer air direction readings were not used. The yaw angle was calculated using the law of 

sines. See Figure 3-1 earlier for variable references.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-14

r

 = r,th  =

⎩

⎨

⎧ sin−1







sin 

w ,  = 1

−sin−1



r

sin 

w ,  = 2

Using the recommended speed range (70-50 mph) and regression fit equation, we found 

that the standard errors for the D values for individual runs ranged from 16 to 76 percent, with a 

median value of 31 percent. The standard error for the A value was even larger, ranging from 25

to 186 percent. These values are extremely high, compared to applying this method through the

full speed range (70-0 mph), where the median of the standard errors for the D values was 

approximately 3 to 4 percent.

In a typical unloaded coastdown from 70 to 50 mph, the average vehicle speed is 

approximately 59 mph. The SAE J1263 and HD Phase 1 limit for cross wind is 5 mph. Under 

this limiting scenario, the average yaw angle experienced during this speed range would be 4.8° 

[tan-1(5/59)]. This does not provide a large enough spread in yaw angle to develop a yaw sweep 

to produce a statistically meaningful wind averaged drag area. Furthermore, if winds are 

relatively constant throughout a test, the constant cross wind would provide yaw angles around 

5° and -5°, without much in between. If winds are not exactly perpendicular, then there would 

be more of a distribution closer to 0°, but with less data at wider angles.

The data collected here show that the bulk of the yaw angles from the data collected 

within Phase 1 wind requirements falls between -4° and 4°. Further analysis shows that 

statistically significant curves of CdA versus yaw angle could not be formed due to the variability

of the data. Since the absolute value of the yaw angle is used in the analysis, this method also 

assumes that the aerodynamic characteristics are symmetrical, which is a not a safe assumption. 

Even using the absolute values, the CdA versus yaw angle curve is not statistically meaningful, as

shown in Figure 3-3.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-15

Figure 3-3 Example data from the high-speed yaw angle analysis shows high uncertainty of the statistical fit. 

The solid line is the yaw curve with the dashed lines representing the upper and lower 95% confidence limits 

of the mean.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-16

After evaluating the methods described above, the agencies are proposing that the highlow iteration analysis method be used to determine the drag area GEM input. In general, while 

coastdown tests are used to determine the aerodynamic drag, the procedure itself is a road load 

procedure, not an aerodynamic one. That is, the measured or calculated forces represent the total 

road load from which the aerodynamic forces must be calculated or inferred; they are not 

measured directly. As a result, all analysis methods that are used in conjunction with 

coastdowns must calculate or infer aerodynamic forces.

3.2.2.2 Specifications and Protocols for Conducting Constant Speed Testing

Similar to the Coastdown Test Procedure, the Constant Speed Test Procedure is 

conducted on road and used to measure the forces acting on the tractor. In contrast to the 

Coastdown Test where the vehicle is accelerated to a set speed and then allowed to coast to a 

lower speed in neutral, the Constant Speed Test is conducted by measuring torque along the 

driveline at various constant speeds. This helps to reduce measurement uncertainty due to 

potential driveline vibration experienced during coastdown and better isolate the force 

contributions between speed transitions over the speed range (e.g., aerodynamic drag dominance 

at high speed; a mix of aero drag and mechanical/frictional forces in middle speeds; and 

mechanical/frictional force dominance at low speeds). In addition, whereas the total force, and 

consequently the total drag force, is derived based on the speed and time for the coastdown test, 

the constant speed test measures the total force at the wheels using wheel hub torque meters 

and/or a driveshaft torque meter. It can also incorporate, where needed and available, engine 

speed and torque, transmission gear ratio, rear axle losses, or other relevant data. The constant 

speed test has the potential to reduce uncertainty compared to a coastdown because it can collect 

data at a single speed for a sustained amount of time. For HD Phase 1, we allowed the use of 

Constant Speed testing as an alternate aerodynamic method but did not promulgate specific test 

procedure requirements. In lieu of this, a manufacturer would have been required to develop its

own test procedure for constant speed testing and submit it to the agencies for approval.

For HD Phase 2, we are proposing specific requirements for conducting the constant 

speed test, to be used by manufacturers choosing this testing method. Accordingly, we evaluated 

the constant speed testing using the same vehicles receiving the coastdown test procedure. For 

our evaluations, we used several speeds to determine the optimal speeds for constant speed 

testing. In addition, we performed the testing with both wheel hub torque meters and a 

driveshaft torque meter to quantify the benefits and detriments of both methods. More details on 

our test set up for constant speed testing and our results are discussed below.

In addition to evaluating the constant speed test procedure, we are also seeking comment

regarding making the constant speed test procedure the reference aerodynamic method. We 

received suggestions to this effect from industry during the HD Phase 1 rulemaking process, 

since some of the OEMs have European controlling interest. Thus, use of the constant speed test 

would allow them to harmonize test procedures with their European counterparts, who are 

required to use constant speed testing. We did not have data to support use of the constant speed 

test as the preferred method for Phase 1. Since then we have evaluated and are proposing 

specific constant speed testing requirements for HD Phase 2. Thus, we are taking this 

opportunity to explore this approach again.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-17

3.2.2.2.1 Constant Speed Test Procedure Specifications

For our evaluation of the constant speed test, we used the following procedure and 

specifications:

 Used four High Resolution Truck Torque Wheel Transducers with approximate 5 lb-ft 

resolution during testing of each of the tractors. Mechanical protection against high 

torque application (both acceleration and braking) telemetry, and associated wheel 

adapters, encoder, amplifier, and power supply was employed.

 Used an in-line strain-gauged torque flange, which was used to measure driveshaft torque 

during the testing each of the tractors (i.e., driveshaft torque sensor). The torque flange 

was an ANSI C12.20 0.5 class meter with a range of 0 to 5,000 Newton- meters (N-m). 

This torque meter utilizes special adapters and cannot be connected directly to the drive 

shaft. A modified drive shaft shall was acquired for each tractor to accommodate the 

torque meter. These drive shafts were dynamically balanced.

 Used a driveshaft torque sensor and wheel hub meter simultaneously to collect data at the 

drive shaft and the wheels for comparison. The driveshaft torque sensor was calibrated 

according to 40 CFR 1065.310.

 During the test, the following parameters were monitored (data collected), similar to the 

coastdown tests:

o Air speed data using an anemometer mounted on the trailer approximately 0.85 m 

above the trailer roof, at the midpoint of the trailer width, at the leading edge of 

the trailer.

o Vehicle speed using an optical fifth wheel.

o Engine speed using the electronic control unit (ECU).

o Grade data along the location of track or road where testing was performed.

 The vehicle was warmed-up by being driven for 30 minutes prior to the test. The same 

road was used for the constant speed testing coastdown to ensure grade/location 

consistency. 

 Testing was performed at the following speeds and durations while recording torque and 

engine data. Cruise control was used to maintain speeds, except for the lower one or two 

speeds for certain tests.

o 10 mph – 7.5 minutes in each direction

o 20 mph – 7.5 minutes in each direction

o 30 mph – 7.5 minutes in each direction

o 50 mph – 8-10 minutes in each direction

o 70 mph – Approximately 11.25 miles or 9.6 minutes in each direction.

 If necessary, multiple passes were conducted.

The agencies conducted constant speed testing through Southwest Research Institute 

along the same stretch of roadway as the coastdown testing.

3.2.2.2.2 Constant Speed Test Procedure Analysis Methodology

For analysis of the constant speed test procedure data, the 10-Hz data were filtered using 

a two-second centered moving average and then split into 10-second segments over which the 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-18

forces, air speed, and air direction were averaged. For tractors equipped with the driveshaft 

torque meter, the road load force was calculated as follows:

, =



 ∙ 

+ 

For tractors also equipped with the wheel torque meters, the road load force was 

calculated as follows:

, =





+ 

Where: 

τshaft = driveshaft torque

ωeng = engine speed

GR = transmission gear ratio

FRL,shaft = road load force calculated from the driveshaft torque

τwheel = wheel torque, sum of all four wheel torque measurements

ωwheel = wheel speed, average of all four wheel speed measurements

FRL,wheel = road load force calculated from the wheel torque

The analysis method involved a force subtraction method where the average road load 

force at 10 mph was assumed to be made up of just rolling resistance and mechanical forces. 

This value was subtracted from each 50-mph and 70-mph road load force measurement to 

estimate aerodynamic forces, and CdA was calculated similar to coastdowns.

 = 

,

 =  −  (50 & 70 mph runs only)

 =

2





Since wind conditions would vary from one test day to another, the yaw angle would also 

change, creating larger yaw angles and different yaw angle spreads for some tests. In order to 

compare the constant speed results to one another, the CdA values were regressed against a 

fourth-order polynomial of yaw angle to determine the zero-yaw CdA. Any tests that were 

conducted outside of the wind speed constraints were included along with tests within the wind 

constraints because the yaw angle distribution helped developed the polynomial fit.

() = (0) +  + 

 +  + 

If over 75 percent of the points occurred at yaw angles between -2 and +2 degrees, then 

the zero-yaw CdA was determined by using the average of the CdA values between those yaw 

angles. This would occur on test days where the prevailing winds were more parallel to the 

direction of travel, resulting in low yaw angles. A regression was not used in this case because 

the F test showed a low level significance of the polynomial fit given above. These regressed or 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-19

averaged zero-yaw CdA values are the constant speed test results that are reported in this 

document.

Since the wheel torque measurements are downstream of the rear axle losses, they are a 

more accurate measurement of actual road load forces than the driveshaft torque measurements. 

The first several tests were conducted without wheel torque meters, so those tests’ results, which 

were determined only through the driveshaft torque measurements, were reduced by 6 percent, 

based on the ratio of CdA determined from one of the vehicles with a similar axle that was 

equipped with both driveshaft and wheel torque meters.

3.2.2.3 Results for Coastdown and Constant Speed Testing Using 

Modified/Specified Test Procedures

Using the coastdown test procedure with modified analysis methodology and the 

specifications and protocols for the constant speed test procedure, we evaluated four Class 8, 

high roof sleeper cabs, one from each of the heavy-duty tractor OEMs, and a Class 8, high roof, 

tandem axle sleeper cab with a 53’ dry box van trailer. The results using these procedures on 

tractor-trailer combinations with the trailer in the standard configuration (e.g., a 53’ dry box van 

with no trailer devices installed) are shown below in Table 3-3 and Table 3-4 for the coastdown 

and constant speed test procedure, respectively.

Table 3-3 Summary of Results from Coastdown Testing

CAB 

TYPE

ROOF 

HEIGHT

TRAILER 

CONFIGURATION

CdA

[m

2

]

STD. 

ERROR

# OF VALID 

RUNS

Sleeper 1 High Standard 5.9 0.5 % 14

Sleeper 2 High Standard 6.2 2.1 % 14

Sleeper 3 High Standard 6.1 2.3 % 14

Based on this test procedure, the results from our constant speed testing are shown in

Table 3-4 below.

Table 3-4 Summary of Results from Constant Speed Testing

CAB 

TYPE

ROOF 

HEIGHT

TRAILER 

CONFIGURATION

CdA

[m

2

]

STD. 

ERROR

Sleeper 1 High Standard 6.1 0.7%

Sleeper 2 High Standard 5.9 1.8%

Sleeper 3 High Standard 5.9 0.3%

The coastdown and constant speed results show a similar range of CdA values. Using 

different speed ranges in the coastdown method or analysis techniques in either method may

produce slightly different CdA ranges, depending on certain factors such as the temperature and 

speed dependence of tire rolling resistance.

The uncertainties, however, are unlikely to change significantly, since they are based on 

run-to-run variability for a given analysis technique. To ensure the required test repeatability
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-20

and method acceptance needed for future certification and compliance purposes, we looked at the 

standard error of the test and the alignment between the test procedures for each tractor. The 

standard error of the constant speed tests was determined through the statistical fit of the tensecond segments through the yaw polynomial where the polynomial fit was used. In this case, 

the standard error represents that of the zero-yaw CdA from that fit. Otherwise, the standard error 

is of the mean of the CdA values between -2 and 2 degrees. This may slightly underestimate the 

uncertainty due to the precision error that may be introduced in the low-speed segments. Other 

trailer configurations tested with coastdowns show lower uncertainties than the standard trailer.

Consequently, either on-road test should be acceptable for aerodynamic assessment. The 

agencies are still collecting data using both test procedures and will continue to evaluate the 

accuracy and repeatability of both.

Accordingly, we are proposing to require the use of the coastdown test procedure as the 

reference method with the high-low iteration analysis method discussed above. We are also

specifying the constant speed aerodynamic assessment test procedure based on our above 

analysis. For constant speed testing, manufacturers may use a driveshaft torque meter or wheel

hub torque meters for constant speed testing provided they meet or exceed the specifications 

given above for each type of device.

We are completing testing on an additional sleeper cab tractor from another OEM and on 

a day cab tractor. Although all of the on-road testing for tractors is not complete, we believe the 

results of future on-road testing will be consistent with the trends emerging from the existing 

data. Once all on-road testing is complete, a full report will be included in the docket and further 

considered in the context of the tractor aerodynamic standards.

3.2.2.4 Modifications to Computational Fluid Dynamics for HD Phase 2

Computational Fluid Dynamics, or CFD, capitalizes on today’s computing power by

modeling a full size vehicle and simulating the flows around this model to examine the fluid 

dynamic properties, in a virtual environment. CFD tools are used to solve either the NavierStokes equations that relate the physical law of conservation of momentum to the flow 

relationship around a body in motion or a static body with fluid in motion around it, or the 

Boltzmann equation that examines fluid mechanics and determines the characteristics of discreet, 

individual particles within a fluid and relates this behavior to the overall dynamics and behavior 

of the fluid. CFD analysis involves several steps: defining the model structure or geometry

based on provided specifications to define the basic model shape; applying a closed surface 

around the structure to define the external model shape (wrapping or surface meshing); dividing 

the control volume, including the model and the surrounding environment, up into smaller, 

discreet shapes (gridding); defining the flow conditions in and out of the control volume and the 

flow relationships within the grid (including eddies and turbulence); and solving the flow 

equations based on the prescribed flow conditions and relationships.

This approach can be beneficial to manufacturers since they can rapidly prototype (e.g., 

design, research, and model) an entire vehicle without investing in material costs; they can 

modify and investigate changes easily; and the data files can be re-used and shared within the 

company or with corporate partners. 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-21

For HD Phase 1, we established some CFD procedures based on our results and industry

collaboration since there were no standardized practices at the time. In addition, to ensure data 

consistency, a minimum set of characteristics and criteria was included for CFD analysis to 

ensure that the boundary and surface conditions are not too coarse and, thus, not representative 

of the real tractor and environmental conditions. 

For HD Phase 2, we are proposing to either use the existing criteria from HD Phase 1 or 

require adherence to a newly established, Society of Automotive Engineering (SAE) standard for 

CFD, SAE J2966.5 Accordingly, we are comparing the criteria we set forth in HD Phase 1 with 

SAE J2966 to assess the efficacy of adopting this new standard.

In addition, we are considering enhancements to the specified conditions for performing

CFD analysis. Specifically, we specified the use of a simulated vehicle speed of 55 miles per 

hour and “ambient conditions consistent with the coastdown test procedures.” These conditions 

are ambiguous and may require greater specificity to ensure that the CFD analysis is more 

closely simulating the real world conditions. As a result, we are seeking comment on the need to 

improve the simulated ambient environmental conditions for conducting CFD analysis.

Also, similar to wind tunnels, we are proposing to modify the measured yaw angles used 

in the CFD analysis to incorporate the concept of the wind average coefficient of drag. Under 

HD Phase 1, we only required manufacturers to perform the CFD analysis at a zero degree yaw 

angle. For HD Phase 2, we are requiring the CFD analysis to be performed at additional yaw 

angles for generating the yaw sweep curve and calculating the wind average coefficient of drag. 

This is discussed further in the following section.

Finally, our CFD specifications do not include or require the use of turbulence intensity

in the CFD analysis, and the use of a turbulence model is only required “if applicable.” As a 

result, there is less ability to capture transient flow phenomena in the CFD analysis. Based on 

developments since HD Phase 1, the ability to include turbulence in the CFD analysis without 

severely impacting the analysis run time has improved. Therefore, we are seeking comment on 

the inclusion of turbulence in CFD analysis. 

As we consider these proposed modifications and a revised standard reference trailer for 

tractor aerodynamic assessment, it is important to evaluate the ability of CFD to characterize the 

aerodynamics when aerodynamic trailer devices are employed. Therefore, we will be evaluating 

trailer aerodynamic devices for CFD method validation to support HD Phase 2.

3.2.3 Aerodynamic Assessment and Use of Wind Averaged Drag Area

Finally, we received comments in HD Phase 1 regarding the use of the wind averaged

coefficient of drag (WACd) since WACd accounts for aerodynamic performance across a broader 

spectrum of wind conditions rather than a pure headwind or tailwind (e.g., zero degree yaw). 

Consequently, the use of WACd for aerodynamic assessment may better reflect real-world 

aerodynamic performance and fuel consumption. Therefore, we assessed the use of WACd for 

HD Phase 2 and the results are discussed below in this section.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-22

EPA and NHTSA recognize that wind conditions have a greater impact on real world 

CO2 emissions and fuel consumption of heavy-duty tractors than light-duty vehicles. As stated 

in the NAS report6, the wind averaged drag coefficient is about 15 percent higher than the zero 

degree coefficient of drag (Cd). The large ratio of the side area of a combination tractor and 

trailer to the frontal area illustrates that winds will have a significant impact on drag. One 

disadvantage of the agencies’ approach to aerodynamic assessment in Phase 1 is that the test 

methods have varying but limited degrees of ability to assess wind conditions. Wind tunnels are 

currently the only demonstrated tool to measure the influence of wind speed and direction on a 

vehicle’s aerodynamic performance. The coastdown test and computational fluid dynamics 

modeling both have limited ability to assess yaw conditions. The constant speed test has the 

potential for yaw angle measurement capability but it is not certain how its ability to measure the 

influence of wind speed and direction on a vehicle’s aerodynamic performance compares to the 

wind tunnel. 

To address this issue in HD Phase 1, the agencies finalized the use of coefficient of drag 

values that represented zero yaw (i.e., representing wind from directly in front of the vehicle, not 

from the side). The agencies recognized that the results of using the zero-yaw approach will 

produce fuel consumption results in the regulatory program which are slightly lower (i.e. predict 

better fuel consumption results) than in-use, but we believed this approach was appropriate since 

not all manufacturers were using wind tunnels for the aerodynamic assessment to the extent 

needed for wind averaged Cd quantification purposes. 

During HD Phase 1, we examined full yaw sweep data from the reduced-scale wind tunnel 

test for three manufacturer 1/8th scale models. Below in Figure 3-4 are the yaw sweep graphs for 

three manufacturers’ vehicles in the reduced-scale wind tunnel with the WACd shown for 

comparison. This graph indicates that, although the zero-yaw Cd results for two tractors can be 

nearly identical at zero yaw, their aerodynamic performance may diverge as the yaw angle is 

increased. As a result, although the two tractors exhibit similar zero-yaw aerodynamic 

performance, their aerodynamic performance in non-zero yaw conditions may be drastically

different and, consequently, so would their real-world fuel efficiency.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-23

Figure 3-4 Full yaw sweeps and wind-average coefficients of drag (WACds) for three manufacturer, 1/8th 

scale, tractor models in the reduced scale wind tunnel.

0.000

0.050

0.100

0.150

0.200

0.250

0.300

-10.000 -8.000 -6.000 -4.000 -2.000 0.000 2.000 4.000 6.000 8.000 10.000

Truck A Yaw Sweep

Truck A WACd

Truck B Yaw Sweep

Truck B WACd

Truck C Yaw Sweep

Truck C WACd
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-24

Table 3-5 below shows the results of an analysis on the impact of improving the non-zero 

yaw performance on the coefficient of drag for Tractor A versus Tractor B and Tractor C, 

respectively.

Table 3-5 Absolute Deltas and Percent Difference for Individual Yaw Points for Tractor A versus Tractor B 

and Tractor C.

+/- YAW 

ANGLE

AVERAGES

TRACTOR A VERSUS 

TRACTOR B

TRACTOR A VERSUS 

TRACTOR C

Delta Cd Delta Cd % 

Difference

Delta Cd Delta Cd % 

Difference

Zero Degree 0.001 0.18% -0.031 -5.84%

One Degree -0.003 -0.53% -0.035 -6.65%

Three Degrees -0.018 -3.33% -0.052 -9.58%

Six Degrees -0.035 -5.85% -0.078 -13.05%

Nine Degrees -0.042 -6.24% -0.098 -14.52%

Based on the results, although Tractor A and Tractor B have similar zero degree 

aerodynamic performance, their aerodynamic performance at higher yaw angles results in a 0.53

to 6.24 percent loss in aerodynamic efficiency. This difference is exacerbated for the case of 

Tractor A versus Tractor C where the zero degree performance of Tractor C is worse than 

Tractor A.

Due to this decreased aerodynamic performance at each individual yaw angle, the WACd

values are impacted as follows when again comparing Tractor A versus Tractor B and Tractor C, 

respectively, as shown in Table 3-6.

Table 3-6 Absolute Deltas and Percent Difference for Wind Averaged Coefficient of Drag (WACd) for 

Tractor A versus Tractor B and Tractor C.

COMPARISON ABSOLUTE 

DELTA

% 

DIFFERENCE

Tractor A vs. B -0.021 -3.5%

Tractor A vs. C -0.069 -11.7%

Consequently, by focusing solely on zero degree yaw angle drag, there is an additional 

3.5 to 11.7 percent of benefit lost due to higher aerodynamic drag at greater yaw angles.

As a result of this data and comments we received during and since HD Phase 1, we are 

proposing the use of additional yaw data to develop a wind-average drag area to be used for 

input into the GEM model and assigning a GHG emissions score. Further, the agencies are 

proposing to require manufacturers to use the yaw sweep calculation in SAE J1252 to determine 

WACd and, coupled with the frontal area, the wind averaged drag area (WACdA).

Specifically, we are proposing to require the CdA data for zero degrees, positive/negative 

one degree, positive/negative three degrees, positive/negative six degrees and positive/negative 

nine degrees to calculate WACdA. This is in accordance with SAE J1252 which requires a 

minimum of six points for the calculating WACd rather than just using zero degrees as is 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-25

currently required for the HD Phase 1 version of GEM input and positive/negative six degrees 

currently used for the generation of the HD Phase 1 WACd as a yaw sweep correction factor. 

This proposed methodology would apply for any aerodynamic method that is used to generate 

the WACd using an approved series of yaw angles. Alternatively, a manufacturer may use 

request to use a series of different angles (e.g., 0, ±2, ±4, ±6, ±8 , ±10 degrees) or fewer angles 

(e.g., ±3, ±6, ±9) with advanced approval by and demonstration to the agencies that the series of 

different/reduced yaw angles provides equivalent results to the required yaw angles.

We will also need to generate a yaw sweep curve and WACdA based on the coastdown 

reference method results to determine the appropriate GEM model inputs. At present, the 

coastdown procedure does not account for the varying wind direction well enough to reliably

generate a yaw sweep curve. Therefore, one approach we are proposing to require is for a 

manufacturer to generate the yaw sweep curve using an alternate method, in particular, the wind 

tunnel or CFD, by taking the differential of the coastdown result and zero-yaw result from the 

alternate method and additively applying the coastdown-alternate method zero yaw to each of the 

points on the alternate method yaw sweep curve. Both of these techniques are demonstrated 

below in Figure 3-5.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-26

Figure 3-5 Proposed development of a coastdown yaw sweep curve based on additive offset from alternate 

method yaw sweep curve.

0.531

0.724

0.801

0.607

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

-10.000 -8.000 -6.000 -4.000 -2.000 0.000 2.000 4.000 6.000 8.000 10.000

Yaw Sweep Curve Shift Concept

Multiplicative Approach

Additive Approach
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-27

Using the results in Figure 3-5 as it applies to the entire yaw sweep curve; a 

multiplicative approach changes the shape of the yaw sweep curve whereas the additive 

approach maintains the yaw sweep curve shape. Therefore, the additive approach is a more 

appropriate to use than the multiplicative approach when using the full yaw sweep curve to 

generate the coastdown yaw sweep curve. This, however, requires the addition of the offset to 

each point on the yaw sweep curve, to shift and maintain the shape of the yaw sweep curve, and 

recalculating the wind average drag area based on the shifted curve.

Alternatively, a simpler approach is to use the offsets between the zero degree yaw and 

the coastdown values to calculate Falt-aero; the zero degree yaw and WACdA values to calculate a 

wind average drag offset from the alternate method; and apply them to the coastdown results 

which reduces the complexity of calculating a virtual coastdown wind average drag area. This 

approach is explored in further detail in Section 3.2.9.

Accordingly, we are proposing to require WACdA as the aerodynamic GEM input for HD

Phase 2 and invite comment on the methodology used to generate a coastdown equivalent 

WACdA. Although the data above is shown in Cd format, we will use WACdA format for alternate 

method results for HD Phase 2. The WACdA values used to support HD Phase 2 are discussed 

below in Section 3.2.9.

3.2.4 HD Phase 1 Yaw Sweep Correction

In this proposal, the agencies propose to correct the Phase 1 yaw sweep correction factor 

equation to create equivalency between the two methods currently allowed by the regulations to 

determine the yaw sweep coefficient of drag area. The HD Phase 1 aerodynamic bins are based 

on zero degrees yaw. However, the current regulations allow manufacturers the option of 

determining the aerodynamic bin levels based on their wind average drag performance relative to 

the nominal wind averaged drag performance of the baseline fleet evaluated in Phase 1. The yaw 

sweep correction factor, as defined currently in 40 CFR 1037.521, allows the determination of 

the CDA based on (1) averaging the measurements at positive six degrees and negative six 

degrees (± 6 degrees) of yaw and (2) a full yaw sweep as defined by SAE J1252. However, 

these two methods do not produce the same wind averaged drag CDA value, as shown below in 

Table 3-7. The CDA based full yaw sweeps produce lower values, on the average of 3.3 percent. 

Table 3-7: CDA Values

CDA (ZERO YAW) CDA (AVG ± 6 DEGREES) CDA (FULL YAW 

SWEEP)

Sleeper Cab 1 5.425 6.239 6.043

Sleeper Cab 2 5.553 6.619 6.405

Sleeper Cab 3 5.622 6.442 6.285

Sleeper Cab 4 5.563 6.663 6.373

The value used to represent the nominal wind average drag performance of the fleet 

developed for Phase 1 was based on ± 6 degree measurement values. In order to remove the 

discrepancy in measurement methods, the agencies developed a new factor for determining the 

yaw sweep correction factor for data based on full yaw sweeps. We propose that manufacturers 

use the following equation when using wind averaged drag values based on the full yaw sweep.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-28

CFys = [CDA from full yaw sweep * 0.8330] / [CDA at zero yaw]

3.2.5 Aerodynamics for Trailers

For HD Phase 2, the agencies are proposing GHG standards reflecting GHG and fuel 

consumption reductions from trailers. Aerodynamic improvements are among the technologies 

on which those proposed standards are predicated. New aerodynamic technologies have been 

implemented on box trailers to improve their aerodynamic efficiency and lower overall tractortrailer fuel consumption. In addition, the agencies have assessed the extent that some of these 

technologies may migrate to the trailer sector without regulation, and the extent these 

improvements should be reflected in the reference trailer used in tractor certification testing. 

Consistent with the proposed trailer regulations, our aerodynamic assessment of different 

trailer configurations (applicable to coastdown, constant speed and reduced-scale wind tunnel

testing) and trailer types (applicable to reduced scale wind tunnel testing only) was limited to 

box van type trailers including dry box, reefer, and pup. Specifics on the applicable trailer types 

and certification protocols are discussed further in Section IV. D(2), of the preamble.

3.2.5.1 On-Road Aerodynamic Assessment of Different Trailer Configurations

We are also proposing to use the on-road test procedures as one method to determine

aerodynamic performance for the proposed trailer program. Specifically, the on-road testing 

would estimate the delta drag area between a tractor-trailer combination for a trailer equipped 

without and with aerodynamic trailer devices (i.e., A to B testing; A constitutes a test without the 

technology; B constitutes a test with the technology). In addition, we used these test procedures

to evaluate the ability of on-road testing to capture the aerodynamics of trailers equipped with

and without aerodynamic devices for the standard trailer used in the tractor program. 

Consequently, we assessed different trailer configurations using the proposed coastdown 

and constant speed test procedures with and without trailer aerodynamic devices installed on the 

trailer. Specifically, we performed coordinated, HD Phase 2 and SmartWay testing on-road for 

53’ dry box van trailers; which represent the bulk of the trailer market. The results of testing 53’ 

dry box van trailers in different configurations are shown below in Table 3-8 and Table 3-9.

Table 3-8 Summary of Results from Coastdown Testing with Different Trailer Configurations

CAB 

TYPE

ROOF 

HEIGHT

CONFIGURATION CdA

[m

2

]

STANDARD 

ERROR

DELTA CdA

(VS. STANDARD)

% DELTA CdA

(VS. STANDARD)

Sleeper 1 High Standard 5.9 0.5% -- --

With Trailer Skirt 5.4 1.0% 0.5 8.5%

With Trailer Skirt and Boat Tail 5.0 1.0% 0.9 15%

Sleeper 2 High Standard 6.2 2.1% -- --

With Trailer Skirt 5.6 1.3% 0.6 10%

With Trailer Skirt and Boat Tail 5.1 1.1% 1.1 18%

Sleeper 3 High Standard 6.1 2.3% -- --

With Trailer Skirt 5.6 1.2% 0.5 8.2%

With Trailer Skirt and Boat Tail 5.4 1.1% 0.7 11%
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-29

Table 3-9 Summary of Results from Constant Speed Testing

CAB 

TYPE

ROOF 

HEIGHT

CONFIGURATION CDA

[m

2

]

STANDARD 

ERROR

DELTA CdA

(VS. STANDARD)

% DELTA CdA

(VS. STANDARD)

Sleeper 1 High Standard 6.1 0.7% -- --

With Trailer Skirt 5.2 2.2% 0.7 12%

With Trailer Skirt and Boat Tail 4.9 0.7% 1.2 20%

Sleeper 2 High Standard 5.9 1.8% -- --

With Trailer Skirt 5.5 0.7% 0.4 6.7%

With Trailer Skirt and Boat Tail 5.1 1.0% 0.8 14%

Sleeper 3 High Standard 5.9 0.3% -- --

With Trailer Skirt 5.7 0.4% 0.2 3.4%

With Trailer Skirt and Boat Tail 5.2 0.6% 0.7 12%

In addition to our on-road testing of 53’ dry box vans, we are coordinating with the 

California Air Resources Board (ARB) and U.S. Department of Energy’s (U.S. DOE) National 

Renewable Energy Laboratory (NREL) to evaluate twin, 28-foot (also known as “pup”) trailers 

using the coastdown and the SAE J1526 Type III7test procedure. Also, we are planning 

additional on-road testing of reefer box van trailers in the same location and identical tractors 

used for our evaluations of 53’ dry box vans.

8

The agencies used the results from the on-road testing of 53’dry box vans to assist in

developing aerodynamic standards for the trailer types covered under this proposed rulemaking. 

Specifically, the delta CdA for trailers configured with various devices will be used to determine 

categories of values for GEM input; as discussed further under Section 3.2.7.

Although all of the on-road testing for different trailer types is not complete at the time of 

proposal, we do have data on other trailer types using alternate aerodynamic methods (e.g., 

reduced-scale wind tunnel) and discussed further below. Therefore, the combination of existing, 

on-road and alternate method data on other trailer types is a good foundation to develop 

aerodynamic standards for other trailer types. Further, we believe the results of future on-road 

testing will be consistent with the trends emerging from the existing data. Once all testing is 

complete, a full report will be included in the docket and further considered in the context of the

trailer aerodynamic standards.

3.2.5.2 Reduced-Scale Wind Tunnel Testing of Different Trailer OEMs, 

Configurations, and Trailer Types

In addition to the on-road coastdown and constant speed procedures presented previously, 

we are proposing to allow additional aerodynamic assessment methods (e.g., wind tunnel, CFD) 

to generate delta CdA values for the HD Phase 2 trailer program. In contrast to the on-road test 

procedures, the wind tunnel provides a stable, controllable environment yielding a more 

repeatable test and allows for more accurate measurement of the aerodynamic impact of tractortrailer modifications. In addition, wind tunnels provide testers with the ability to yaw the vehicle 

in a controlled, specific manner at positive and negative angles relative to the original centerline 

of the vehicle to accurately capture the influence of non-uniform wind direction on the Cd (e.g., 

wind averaged Cd). Most trailers in the U.S. are 28’ or longer and the agencies are not aware of 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-30

any viable, available wind tunnels that can accommodate full scale tractor-trailer combination 

vehicles. As a result, we exclusively used reduced-scale wind tunnel (RSWT) testing to support 

the HD Phase 2 aerodynamic assessment rather than a mix of full scale wind tunnel (FSWT) and 

RSWT testing as performed in HD Phase 1.

For HD Phase 2, we are proposing to carry over the use of SAE J1252 with the 

exceptions and modifications noted above. In addition, since the finalization of HD Phase 1, 

SAE J1252 has been updated and we are incorporating by reference the most recent version for 

HD Phase 2. For HD Phase 2, we are performing wind tunnel testing to inform revisions to the 

standard trailer test article for the tractor program and to develop separate trailer standards.

The RSWT testing was performed at the Automotive Research Center (ARC) in 

Indianapolis Indiana. The ARC wind tunnel is a closed single return tunnel with 3/4 open-jet 

working section and moving ground plane (2.3 m wide x 2.1 m high x 5.5 m long (7.5 ft x 6.8 ft 

x 18 ft)). It is powered by an air-cooled 373kW (274 hp) variable speed DC motor that drives a 

9-bladed fan with carbon fiber blades. Its speed may be varied and set at any value from 0 to 

610 rpm. The maximum wind speed is about 50 m/s (164 ft/s). The wind tunnel can 

accommodate a model up to 50 percent scale (1/2 scale) for race car applications down to 12.5

percent scale (1/8th scale) for Class 8 tractor and trailer combinations. The wind tunnel is 

equipped with a moving ground plane (i.e., rolling road), four-stage boundary layer suction 

system, and a top-mounting “Sting” system that allows for yawing of the model. For model 

development, ARC has in-house model developers and can create highly detailed scale models 

using original computer aided design and engineering (CAD/CAE) drawings or using in-house 

scanning equipment to perform scanning and digitizing to create CAD/CAE drawings (see

Figure 3-6 below).
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-31

Figure 3-6 1/8th scale tractor-trailer model in ARC reduced scale wind tunnel.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-32

The RSWT testing was conducted simulating an actual vehicle speed of 65 miles per hour 

(mph) using a tunnel speed of 111.8 mph and Reynolds number (Re) of greater than one million 

(1x106) on one eighth scale models of heavy-duty, Class 8 sleeper and day cab tractors equipped 

with the full aerodynamics package components sold on the full size version of the tractor; 

consistent with our HD regulatory requirements. For our test program, we assumed a base 

tractor-trailer gap of 45 inches and a bogey position of 40 feet (California position) from the 

leading edge of the trailer. 

To support HD Phase 2, we tested the latest model year tractor available from each of the 

four tractor OEMs in sleeper cab form and one tractor OEM in day cab form. The tractor models 

used in the RSWT matched the tractor models used for the on-road testing to the extent feasible. 

For one manufacturer, we were not able to match the exact tractor model but did test models

from that OEM in the RSWT and on-road. While the results across the tractor models are not 

directly comparable, the data provides some representativeness of the expected performance 

from that OEM’s tractors.

In addition, we also tested three 53 foot dry box van trailers from three different trailer 

OEMs: Wabash, Great Dane, and Hyundai Translead; to evaluate aerodynamic trailer devices. 

For aerodynamic trailer devices, we focused on technologies that may improve areas on the 

tractor-trailer where large amounts of aerodynamic drag can occur and tested: one OEM trailer 

front treatment (e.g., front end trailer gap reduction device), two OEM side treatments (e.g., 

trailer side skirts) and one OEM aft treatment (e.g., trailer boat tail). In the case of the trailer 

side skirts and boat tail, a portion of the trailer devices used in the RSWT matched those used for 

the on-road testing. Thus, we ensured some overlap between the RSWT and on-road testing,

although we were able to test a broader range of trailer devices.

Below in Table 3-10 are the RSWT results for the various configurations of four OEM 

sleeper cabs, one OEM day cab, three OEM 53-foot dry box van trailers, and four aerodynamic 

trailer devices (two trailer skirts, one rear boat tail, one front trailer gap reducer); averaged across 

trailer OEMs. The variation across the three OEM 53-foot dry box van trailers was small and 

should be considered negligible at 1.0 percent on average. The variation ranged from 0.5 to 0.9

percent for three of the four sleeper cab tractors and the day cab and a high of 2.3 percent for one 

of the four sleeper cab tractors. Where there were multiple runs on the same tractor, trailer, and 

device, the results were averaged across those runs and then included in the overall average.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-33

Table 3-10 Results of Reduced-Scale Wind Tunnel Testing to Assess Tractor Trailer Performance with 

Various Aerodynamic Trailer Devices.

CAB TYPE ROOF 

HEIGHT

CONFIGURATION CdA

[m

2

]

DELTA CdA

(VS. STANDARD)

% DELTA CdA

(VS. STANDARD)

Sleeper 1 High Standard 5.425 -- --

With Trailer Skirt #1 5.002 0.423 7.8%

With Trailer Skirt #2 4.914 0.511 9.4%

With Boat Tail 5.085 0.340 6.3%

With Trailer Gap Reducer 5.450 -0.024 -0.5%

With Trailer Skirt #2 and 

Boat Tail

4.453 0.972 17.9%

With Trailer Skirt #2, 

Boat Tail, and Trailer Gap 

Reducer

4.460 0.965 17.8%

Sleeper 2 High Standard 5.556 -- --

With Trailer Skirt #1 5.054 0.586 10.5%

With Trailer Skirt #2 4.962 0.594 10.7%

With Boat Tail 5.191 0.365 6.6%

With Trailer Gap Reducer 5.565 -0.020 -0.4%

With Trailer Skirt #2 and 

Boat Tail

4.464 1.092 19.6%

With Trailer Skirt #2, 

Boat Tail, and Trailer Gap 

Reducer

4.449 1.106 19.9%

Sleeper 3 High Standard 5.622 -- --

With Trailer Skirt #1 5.145 0.477 8.5%

With Trailer Skirt #2 5.056 0.566 10.1%

With Boat Tail 5.305 0.318 5.6%

With Trailer Gap Reducer 5.632 -0.009 -0.2%

With Trailer Skirt #2 and 

Boat Tail

4.566 1.056 18.8%

With Trailer Skirt #2, 

Boat Tail, and Trailer Gap 

Reducer

4.560 1.062 18.9%

Sleeper 4 High Standard 5.563 -- --

With Trailer Skirt #1 5.079 0.481 8.7%

With Trailer Skirt #2 5.003 0.557 10.1%

With Boat Tail 5.229 0.332 6.0%

With Trailer Gap Reducer 5.547 0.015 -0.3%

With Trailer Skirt #2 and 

Boat Tail

4.436 1.121 20.2%

With Trailer Skirt #2, 

Boat Tail, and Trailer Gap 

Reducer

4.445 1.112 20.1%

Day High Standard 5.609 -- --

With Trailer Skirt #1 5.353 0.256 4.6%

With Trailer Skirt #2 5.298 0.311 5.6%

With Boat Tail 5.240 0.370 6.6%

With Trailer Gap Reducer 5.596 0.014 0.2%
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-34

With Trailer Skirt #2 and 

Boat Tail

4.910 0.699 12.5%

With Trailer Skirt #2, 

Boat Tail, and Trailer Gap 

Reducer

4.902 0.707 12.6%

We believe that most trailers in the U.S. fleet will adopt some aerodynamic technologies 

by 2021. We are proposing to revise the standard trailer definition for 53-foot air ride dry box 

van trailers to include aerodynamic trailer devices. Specifically, we are proposing trailer skirts 

as part of the standard configuration for tractor aerodynamic evaluation. We are using the results 

in Table 3-10, to inform the proposed tractor aerodynamic bin categories for use as GEM inputs.

In addition to evaluating the aerodynamics of 53 foot dry box vans in the RSWT, we also 

evaluated the aerodynamics of shorter, 28 foot dry box van trailers, singular or in

tandem/dual/twin configuration, known as “pup” trailers. This testing was performed using a 

single-axle, day cab tractor using similar trailer devices (e.g., trailer skirts, rear boat tail, and 

front trailer gap reducer) with a tractor-to-first trailer gap of 44 inches and a first pup-to-second 

pup trailer gap of 48 inches. Table 3-11 shows the results of this testing for the configurations 

tested:

Table 3-11 Results of Reduced-Scale Wind Tunnel Testing to Assess Tractor Trailer Performance with 

Various Aerodynamic Trailer Devices.

DUAL PUP TRAILER RESULTS

Configuration CdA

[m2]

Delta CdA

(vs. Standard)

% Delta CdA

(vs. Standard)

Standard 6.022 -- --

Skirt on First Trailer 

Only

5.902 0.120 1.99%

Skirt on Second 

Trailer Only

5.735 0.286 4.76%

Skirt on Both 

Trailers

5.586 0.436 7.24%

Skirt and Gap 

Reducers on Both 

Trailers 

5.289 0.733 12.2%

Skirt and Gap 

Reducers on Both 

Trailers w/ tail on 

second trailer

5.208 0.814 13.5%

SINGLE PUP TRAILER RESULTS

Standard 5.384 -- --

Skirt on Trailer 5.226 0.159 2.95%

Skirt and Gap 

Reducer on Trailer

4.971 0.414 7.68%

Based on the results in Table 3-11 above, the potential to significantly improve the 

aerodynamic performance of pup trailers, singular or in tandem/dual/twin, exists using various 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-35

trailer devices. The maximum package of trailer side skirts on both trailers, gap reducers on both 

trailers, and a boat tail on the second trailer produced a benefit of 13.5 percent over the standard 

dual pup trailer configuration.

However, since pup trailers may be frequently interchanged (e.g., drop off the second pup 

trailer or both pup trailers at one destination; proceed to the next destination to pick up a new 

first pup trailer, second pup trailer or both pup trailers), the most practical application may be 

dual pup trailers with just trailer side skirts and gap reducers on both trailers. The presence of 

boat tail/aft treatment restricts that trailer to a second/rear position, depending on the length of 

the aft trailer device, and omitting this device allows for pup trailer symmetry. Even with the 

omission of an aft trailer device, the trailer side skirts and gap reducers on both trailers 

configuration still achieves a benefit of 12.2 percent. Notwithstanding, the added use of aft 

trailer devices; with proper consideration for the length of the device to accommodate 

operational constraints versus the potential tradeoff in aerodynamic performance with reduced, 

aft trailer device length; should be investigated further to identify additional dual pup trailer 

optimization.

Installing skirts/gap reducers on both trailers for dual pup trailer configurations also 

produces benefits from the use of a single pup trailer with trailers side skirts and gap reducer; 

achieving a benefit of 7.68 percent versus the standard, single pup trailer without any devices. 

There may be occasions where a dual pup tractor-trailer must travel between destinations in 

single pup trailer configuration. Thus, the presence of trailer side skirts and gap reducers on both

pup trailers also aids the aerodynamic performance for the occasions where it is operated in a 

single pup trailer configuration. 

Finally, we also evaluated the impact of aerodynamic devices on reefer trailer 

performance. Reefer trailers are similar in basic shape to 53-foot dry box van trailers, but are 

equipped with a thermal refrigeration unit (TRU) on the front of the trailer. Thus, the TRU fills 

the gap space between the tractor-trailer where gap reduction technology would normally be 

fitted. In addition, based on conversations with reefer trailer manufacturers, although reefer 

trailers are less prevalent in the field, they tend to see much more operation than the typical 53-

foot dry box van trailer due to their smaller numbers (1.5-2.2 to 1 for tractor to reefer trailer ratio 

versus 3 to 1 for conventional 53-foot dry box van trailers). As such, there is more opportunity

to realize the benefits from optimizing reefer trailers.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-36

Table 3-12 Results of Reduced-Scale Wind Tunnel Testing of Reefer Trailer Using a 2012 Model Year 

Tractor

CONFIGURATION CDA

(ZERO 

YAW) [m

2

]

DELTA CdA

FROM 

STANDARD 

REEFER (% 

DELTA)

DELTA CdA VS.

STANDARD DRY 

BOX VAN TRAILER 

FROM THE SAME 

TRAILER OEM

% DELTA (VS.

STANDARD 

TRAILER)

Standard Reefer 5.840 -- -0.264 -4.7%

Skirt Only 5.141 0.699 (12.0%) -0.196 -4.0%

Boat tail only 5.607 0.232 (4.0%) -0.448 -8.7%

Skirt +Boat Tail 4.596 1.244 (21.3%) -0.114 -2.5%

Vs. OEM standard 

trailer w/ Skirt +

Boat Tail + Gap

-- -- -0.146 -3.3%

The results in Table 3-10, Table 3-11, and Table 3-12 will also be used to support the

newly proposed, trailer regulations. Specifically, the delta CdA for trailers configured with 

various devices will be used to determine categories of values for GEM input; as discussed 

further in Section 3.2.7.

3.2.6 Standardized Trailer Definitions for Heavy-Duty Tractor Testing

In HD Phase 1, we finalized the use of a model input (i.e., an input to GEM at 

certification) reflecting a standardized trailer for each subcategory of the Class 7/8 tractors based 

on tractor roof height. The height of the roof fairing is designed to minimize the height 

differential between the tractor and typical trailer to reduce the air flow disruption. Low roof 

tractors are designed to carry flatbed or low-boy trailers. Mid roof tractors are designed to carry

tanker and bulk carrier trailers. High roof tractors are designed to optimally pull box trailers. 

However, we recognize that during actual operation tractors sometimes pull trailers that do not 

provide the optimal roof height that matches the tractor. In order to assess how often tractor and 

trailer mismatches are found in operation, EPA conducted a study based on observations of 

traffic across the U.S.9 Data was gathered on over 4,000 tractor-trailer combinations using 33 

live traffic cameras in 22 states across the United States. Approximately 95 percent of tractors 

were “matched” – i.e. optimized – per our definition (e.g. box trailers were pulled by high roof 

tractors and flatbed trailers were pulled with low roof tractors). The amount of mismatch varied 

depending on the type of location. Over 99 percent of the tractors were observed to be in 

matched configuration in Indiana at the I-80/I-94/I-65 interchange, which is representative of 

long-haul operation. On the other hand, only about 90 percent of the tractors were matched with 

the appropriate trailer in metro New York City, where all mismatches consisted of a day cab and 

a tall container trailer. The study also found that approximately 3 percent of the tractors were 

traveling without a trailer or with an empty flatbed. The agencies therefore concluded in Phase 1

that given this very limited degree of mismatch, it is reasonable to use a standardized definition 

which optimizes tractor-trailer matching. For purposes of compliance testing, the agencies also 

finalized bob-tail testing for low roof and mid roof tractors to facilitate repeatability and 

reproducibility of test data in response to concerns raised by tractor manufacturers.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-37

Recently, trailer OEMs and aftermarket manufacturers have begun implementing new 

aerodynamic technologies on box trailers, to improve their aerodynamic efficiency and to lower 

overall tractor-trailer fuel consumption. There are a range of technologies to affix to the front 

(e.g., nose cones, gap reducers, front lip devices); sides and underbody (e.g., skirts, wedge 

shaped devices aft of trailer landing gear and fore of trailer rear axles/bogeys); top (e.g., vortex 

generating devices to promote turbulent flow and delay boundary layer separation along the 

length of the trailer) and rear (e.g., tails, rear lip spoilers and diffusers) of the trailer. However, 

the most widely implemented devices in the market today are trailer side skirts that extend in the 

gap between the fifth wheel and the trailer bogey to restrict underbody air flow and subsequent 

drag and rear/aft trailer treatments extending from the rear of the trailer (e.g., boat tails, that 

reduce the low pressure region, and associated, subsequent drag, on the rear of the tractortrailer). As discussed in Section III.E(2)(a)(iii) and Section IV.D(2) of the preamble, we estimate 

that approximately 50 percent of the new trailers sold in 2018 will have trailer side skirts.10,11 As 

the agencies are proposing tractor standards for model year 2021 and beyond, we believe that it 

is appropriate to update the standardized box trailer definition to reflect the technologies we 

project will be used on the majority of the trailers in the fleet during that timeframe. Therefore, 

we are proposing for Phase 2 that the standardized box trailer used for tractor certification be 

updated to include a trailer skirt starting in model year 2021. Although we are proposing GHG 

standards for trailers that capture the drag area improvement for various trailer devices, including 

trailer skirts, beginning in model year 2018, this proposed update to include a test article to the 

standardized trailer is strictly for the purpose of certifying tractors beginning in model year 2021, 

and is not related to the proposed trailer GHG standards themselves. 

Based on the test results shown above in Table 3-10, Table 3-11, and Table 3-12 and 

comments from stakeholders, the agencies have chosen to update the test article specifications 

for 53’ dry box van trailers below in Table 3-13. Specifically, the standardized, 53’ air ride dry

van test article we are proposing for Phase 2 will include trailer side skirts on the standardized 

trailer for tractor aerodynamic assessment. We propose that the skirt meet the dimensions shown 

in Table 3-13 based on the devices used for the agencies’ aerodynamic assessment, accounting 

for potential design, differences, variations and tolerances for other trailer attributes (e.g., 

landing gear overlap, rear bogey clearance, ground clearance). Trailer side skirts that do not 

meet the size specifications (e.g., length and width) below can be used, with prior approval from 

the agencies , on the standardized trailer used for the aerodynamic assessment of tractors,

provided that they do not exceed the dimensions in Table 3-13 but are no shorter than 270 inches 

± 4 inches in length when measured along the top edge of the trailer side skirt; and meet all other 

specifications for positioning on the trailer; except for the distance from the front and rear trailer 

edge which may vary based on a shorter skirt length. Finally, the tractor must be a stock OEM 

tractor without additional devices installed, unless devices are included as part of a stock OEM 

aero package (e.g., drive axle wheel covers, fairings or fenders).
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-38

Table 3-13 53’ Dry Box Van Trailer Test Article

53’ AIR RIDE DRY BOX VAN TRAILER

Length: 53 feet (636 inches; 1615.44 cm) ± 1 inch

Width: 102 inches ± 0.5 inches

Height: 102 inches (162 inches or 13 feet, 6 inches (+0.0 inch/-1 inch) from the 

ground)

Capacity: 3,800 cubic feet

Assumed trailer load/capacity: 45,000 lbs

Suspension: Any (see “trailer ride height” below)

Corners: Rounded with a radius of 5.5 inches ± 0.5 inches

Bogie/Rear Axle Position: Tandem axle (std), 146 inches ± 4.0 inches from rear axle centerline to 

rear of trailer. Set to California position.

Skin: Generally smooth with flush rivets

Scuff band: Generally smooth, flush with sides (protruding ≤ 1/8 inch)

Wheels: 22.5 inches. Duals. Std mud flaps.

Doors: Swing doors.

Undercarriage/Landing Gear: Std landing gear, no storage boxes, no tire storage, 105 inches ± 4.0 

inches from front of trailer to centerline of landing gear

Underride Guard Equipped in accordance with per 49 CFR 393.86

Aerodynamic Trailer Device 

Type:

Trailer Side Skirts

Aerodynamic Trailer Device 

Specifications Applicable to 

Trailer Side Skirts:

Skirts must be installed on both sides of the trailer

Skirts must be designed to fit between the landing gear and the rear 

bogey, in the most forward position (California), of the trailer and must 

be:

118 inches ± 4 inches measured from the front of the trailer to the 

leading, forward-most point of the upper/top edge of the skirt

Skirts must be straight and flush with the trailer sides

Skirts must be: 

341 inches, ± 4 inches in total length, measured along the top/upper edge

The same total length on the bottom/lower edge as the top/upper edge 

(e.g., rectangular in shape) but shall be no less than 268 inches ± 4 inches 

in length along the bottom edge (e.g., angled front/rear edge or front

edge only)

36 inches ± 2 inches in total width, measured between the top/upper and 

bottom/lower edge, at the midpoint of the skirt

Skirts may minimally (e.g., 10% or less) overlap the trailer landing gear 

at the forward-most point along the upper/top edge but should not cover 

the trailer landing gear with any portion of the skirt leading/front edge 

(e.g., skirt may not extend forward beyond the landing gear position

Tires for the Standard Trailer and the Tractor:

a. Size: 295/75R22.5 or 275/80R22.5

b. Crr<5.1 kg/metric ton

c. Broken in per Section 8.1 of SAE J1263

d. Pressure per Section 8.5 of SAE J1263

e. No uneven wear

f. No re-treads

g. If these tires or appropriate Smart Way tires are unavailable, the Administrator testing may include tires 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-39

used by the manufacturer for certification.

Test Conditions:

Tractor-trailer gap: 45 inches ± 2.0 inches.

King pin setting: 36 inches ± 0.5 inches from front of trailer to king pin center line.

Trailer ride height: 115 inches ±1.0 inches from top of trailer to fifth wheel plate, measured at the front of 

the trailer, and set within trailer height boundary from ground as described above.

Mud flaps: Positioned immediately following wheels of last axle.

For special cases in which a trailer would be used for coastdown testing for mid-roof and low-roof tractors, 

the specifications for the tanker and flatbed trailers are being carried over from HD Phase 1, and are shown, 

unchanged, in Table 3-14 and 

Table 3-15 below, respectively.

Table 3-14 Tanker Trailer Specifications for Special Testing

TANKER TRAILER

Length: 42 feet ± 1 foot, overall

40 feet ± 1 foot, tank 

Width: 96 inches ± 2 

Height: 140 inches 

(overall, from ground)

Capacity: 7,000 gallons

Suspension: Any (see “trailer ride height” below)

Tank: Generally cylindrical with rounded ends.

Bogie: Tandem axle (std). Set to furthest rear position.

Skin: Generally smooth

Structures: (1) Centered, manhole (20 inch opening), (1) ladder generally centered 

on side, (1) walkway (extends lengthwise)

Wheel fairings:

Wheels: 24.5 inches. Double wide. 

Tanker Operation Empty

Table 3-15 Flatbed Trailer Specifications for Special Testing

FLATBED TRAILER

Length 53 feet

Width 102 inches

Flatbed Deck Heights: Front: 60 inches ± 0.5 inches

Rear: 55 inches ± 0.5 inches

Wheels / Tires 22.5 inch diameter tire with steel or aluminum wheels

Bogie Tandem axles, may be in “spread” configuration up to 10 feet ± 2 

inches.

Air suspension 

Load Profile: 25 inches from the centerline to either side of the load; 

Mounted 4.5 inches above the deck.

Load height 31.5 inches above the load support. 

Trailer should be empty.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-40

The regulations in 40 CFR 1037.501 prescribe the standardized trailer for each tractor 

subcategory (low, mid, and high roof) including trailer dimensions based on the tables above.

3.2.7 Standardized Tractor Definition for Heavy-Duty Trailer Testing

Similar to the standardized trailer definition for tractor aerodynamic assessment, the 

agencies are proposing to define a standardized tractor definition for trailer aerodynamic 

assessment. The proposed standardized tractor definition is based on tractor attributes such as a 

Class 8, high roof, tandem axle tractor equipped with, at a minimum, a roof fairing, cab side 

extenders and fuel tank/chassis skirts. This type of tractor typically meets a Bin III or better 

tractor aerodynamic level under HD GHG Phase 1 and is expected to meet the proposed Bin III 

level for HD GHG Phase 2. We believe the majority of tractors in the U.S. trucking fleet will be 

Bin III or better in the timeframe of this rulemaking and trailer manufacturers have the option to 

choose higher-performing tractors in later years as tractor technology improves. As with the 

standardized trailer’s test article specification, the aerodynamic specification for the standardized 

tractor here is strictly for the purpose of certifying trailers beginning in model year 2018 model 

year. Therefore, although the aerodynamic level of the standardized tractor for trailer 

certification potentially overlaps with tractors designed to meet the 2021 and beyond tractor 

GHG standards, it is only intended to serve as a specification for trailer certification to the 2018 

and beyond trailer GHG standards.

Accordingly, we are proposing that trailer manufacturers would use this standardized 

tractor definition with their trailers to conduct A to B testing to capture the delta CdA for their 

trailers that are either: equipped with aerodynamic devices to meet the proposed trailer standards

or are designed to be more aerodynamic than current, standard trailers. Specifically, the trailer 

manufacturers would use the standardized tractor to generate A to B test values where the “A” 

represents a standard test and “B” represents the modified/advanced trailer; both tests performed 

using the same standardized tractor. 

Subsequently, the tractor manufacturer would input their trailer OEM-specific delta CdA

value in the GEM model and the model would determine the appropriate, default trailer delta 

CdA GEM value; where the default delta CdA GEM value is based on the test results in Sections

3.2.5.1 and 3.2.5.2 above. This value is applied to a default tractor/trailer CdA value based on a 

Bin III or greater Class 8 sleeper cab tractors with a standard trailer. Finally, the default 

tractor/trailer CdA value with the default trailer CdA value applied would be used to determine the 

greenhouse gas emissions for this configuration by the GEM model.

For trailer OEMs that are certifying a trailer where devices are added to an existing OEM 

trailer design, the trailer used for both “A” and “B” test uses a trailer meeting our standardized 

trailer definition for 53’ air ride dry box vans shown above in Section 3.2.6, Table 3-13, without 

any trailer devices installed; with the same standard reference tractor used for both tests.

In contrast, for trailer OEMs that certify a completely new trailer design, the “A” test 

uses a trailer meeting our standardized trailer definition for 53’ air ride dry box vans shown 

above in Section 3.2.6, Table 3-13, without any trailer devices installed and the “B” test would 

be the new, OEM trailer design; with a standard reference tractor used for both tests. In 

summary, the standard reference tractor would be used for all trailer OEM “component” level 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-41

testing; where the “component” in the B test can range from an add-on trailer device up to a 

completely different trailer design.

To assist in defining the standardized tractor for different trailer types, below are the 

proposed trailer sub-categories used for GEM from Chapter IV of the preamble.

Table 3-16 Description of Baseline Tractor-Trailers Used In GEM from Section IV. D(2)(b)(ii), of the 

Preamble

TRAILER SUBCATEGORY FEATURES

Dry van 50 feet and shorter Class 8 high roof day cab, pulling solo 28’ dry van

CdA = 6.1, Crr = 6.0 kg/ton

Dry van longer than 50 feet Class 8 high roof sleeper cab pulling a solo 53’ dry van

CdA = 6.2, Crr = 6.0 kg/ton

Refrigerated van 50 feet and shorter Class 8 high roof day cab pulling a solo 28’ ref van

CdA = 6.0, Crr = 6.0 kg/ton

Refrigerated van longer than 50 feet Class 8 high roof sleeper cab pulling a solo 53’ ref van

CdA = 6.1, Crr = 6.0 kg/ton

Based on this table, we are proposing standardized tractor definitions based on tractor 

type and attributes that reflect the types of tractors used for trailers in each of these 

subcategories. 

Specifically, we are proposing that all tractors for all trailers greater than 50 feet shall use 

a standardized tractor meeting the following criteria for A to B testing: a Class 8, high-roof

sleeper cab, tandem axle tractor that meets HD Phase 2 Bin III or better Class 8 high roof sleeper 

cab tractor aerodynamic standards. For all trailers less than 50 feet, a standardized tractor 

meeting the following criteria shall be used for A-B testing: Class 8, high-roof day cab, dual axle 

tractor that meets HD Phase 2 Bin III or better Class 8 high roof day cab tractor aerodynamic 

standards.

Table 3-17 Characteristics of Standard Tractor for Aerodynamic Assessment of Trailers 

BOX TRAILERS 50 FEET AND 

LONGER

CLASS 8 HIGH ROOF SLEEPER 

CAB

DUAL-AXLE

BIN III OR BETTER TRACTOR, 

CDA < 6.5

CAB SIDE EXTENDERS

FUEL TANK COVER/CHASSIS 

SKIRTS

ROOF FAIRING

Box trailers shorter than 50 feet Class 8 high roof day cab

Dual-axle

Bin III or better tractor, CdA <6.7

Cab Side extenders

Fuel tank covers/Chassis Skirts

Roof Fairing
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-42

3.2.8 Continued Use of the Aerodynamic Alternate Method Factor (Falt-aero) for HD

Phase 2 Tractors

 As the agencies showed in Phase 1, aerodynamic test methods differed in their 

predictions of drag coefficient.

12 On-road methods, such as coastdown and constant speed tests,

are performed in uncontrolled real-world environments, whereas wind tunnel testing is 

performed in constrained, controlled conditions, and CFD is a simulation that attempts to 

replicate complex aerodynamic events. Different test methods have differences with regards to 

environmental conditions, assumptions for non-aerodynamic drag forces, tunnel geometry, 

boundary conditions, and simulation characteristics. These differences can lead to different 

results, even though they are used to measure or calculate the same parameter. The agencies

acknowledged that there will never be perfect alignment between the predicted drag area values

from the aerodynamic methods even with full, appropriate correction for every factor, but 

wanted to allow the use of these methods, which are currently being used by the manufacturers, 

to limit test burden for certification

As a result, for HD Phase 1, we employed the use of an aerodynamic method adjustment 

factor (or Falt-aero) factor, to relate the results from the reference method, a coastdown test, to the 

results from the alternative method as a ratio of the coastdown result to the alternate method 

result for selected Class 8 high roof sleeper cab. The Falt-aero is then multiplied to the results 

generated using the alternative method for all other OEM configurations This allowed 

manufacturers the convenience and lower test burden of using existing aerodynamic protocols 

rather than pursuing extensive data correction to produce equivalent results across the 

aerodynamic methods.

For HD Phase 2, we are proposing to continue to allow the use of data from alternate

aerodynamic test methods, and subsequently the aerodynamic method adjustment factor. Thus, 

for HD GHG Phase 2, we explored the level of agreement between the aerodynamic methods 

similar to HD GHG Phase 1. The method comparison was used in HD GHG Phase 1 to 

demonstrate that for a given tractor and trailer model with specified conditions for each of the 

methods, testing meeting these specifications performed with the required level of precision 

(e.g., repeated with a low level of error) could yield a reasonable level of agreement across the 

aerodynamic method. Consequently, there is no guarantee that the same level of agreement 

would be produced using different facilities, conditions, or test articles. However, although this 

level of agreement is only applicable for that tractor and trailer model using the specified 

conditions for those tools in a specific facility, this data was sufficient to demonstrate the level of 

agreement possible and support the use of specified conditions in HD GHG Phase 1 (as shown in 

Figure 3-7 a and b).
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-43

These trends were similarly observed in the HD GHG Phase 2 data. Accordingly, below in 

Figure 3-7a is the aerodynamic method comparison using HD Phase 2 test results. The 

results are shown are for three of the four Class 8, high roof, sleeper cab tractors from 

coastdown, constant speed and zero degree CdA RSWT testing; normalized to the coastdown 

results. The data in Figure 3-7a is presented using the zero yaw Cd values for the reduced scale 

wind tunnel (RSWT) and the constant speed test results normalized to the coastdown test results 

(e.g., RSWT or constant speed results divided by the coastdown result) similar to the comparison 

used for HD GHG Phase 1 for a standard trailer. The HD GHG Phase 2 data shows a similar 

level of method agreement as HD GHG Phase 1 for the coastdown test and RSWT (constant 

speed testing was not performed for HD GHG Phase 1).

1.00 1.00 1.00

1.03

0.95

0.97

0.92

0.90 0.93

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Sleeper 1 Sleeper 2 Sleeper 3

Coastdown

Constant Speed

RSWT

(Cd*A @ zero yaw)
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-44

Figure 3-7a Method Comparison for Coastdown, Constant Speed and RSWT (Zero-Yaw Cd) Testing 

Normalized to the Coastdown Results for Class 8 High Roof Sleeper Cab Tractors and Standard Trailers

following a HD GHG Phase 1 approach. (Note: The results of Sleeper 1 and Sleeper 3 are not directly 

comparable since the test articles do not match but are shown to represent the overall industry trend given a 

manufacturer’s fleet variance.)

1.00 1.00 1.00

1.03

0.95

0.97

0.92

0.90 0.93

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Sleeper 1 Sleeper 2 Sleeper 3

Coastdown

Constant Speed

RSWT

(Cd*A @ zero yaw)
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-45

It is interesting to note that if the WACd concept had been adopted in HD GHG Phase 1, 

the level of method agreement, and consequently the Falt-aero factor used to characterize and 

account for method inequality, may have been closer to parity (e.g., Falt-aero closer to 1) based 

on the HD GHG Phase 2 WACd data for a standard trailer, as shown in Figure 3-8b below.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-46

Figure 3-8b Method Comparison for Coastdown, Constant Speed and RSWT (WACdA @ 55 mph w/ 7 mph 

wind) Testing Normalized to the Coastdown Results for Class 8 High Roof Sleeper Cab Tractors and 

Standard Trailers. This approach was recommended by commenters for HD GHG Phase 1. (Note: The 

results of Sleeper 1 and Sleeper 3 are not directly comparable since the test articles do not match but are 

shown to represent the overall industry trend given a manufacturer’s fleet variance.)

1.00 1.00 1.00

1.03

0.95

0.97

1.025 1.033 1.035

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Sleeper 1 Sleeper 2 Sleeper 3

Coastdown

Constant Speed

RSWT

(Cd*A @ zero yaw)
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-47

For HD GHG Phase 2, we are proposing to include the wind average drag and will 

discuss this in Section 3.2.9. Further, we will revisit the method comparison with the inclusion 

of wind average drag for a trailer equipped with side skirts; consistent with the proposed HD 

GHG Phase 2 approach. 

3.2.9 Wind-Averaged Drag Area and General Aerodynamic Assessment Results 

Based on Proposed HD Phase 2 Tractor Modifications

For HD Phase 2, we are proposing to use the WACdA as the main GEM input (Section 

3.2.3), revise the standard reference trailer for tractors (Chapter 3.2.6), and continue to use a Faltaero when alternative aerodynamic methods are used for certification and compliance 

demonstration (Chapter 3.2.8). Based on the proposed modifications, this section uses the HD

Phase 2 WACdA results from RSWT testing for a tractor-trailer combination with trailer side 

skirts installed to demonstrate the coastdown yaw sweep shift concept, calculation of the 

alternate method factor (Falt-aero), and provide a cross-method comparison for illustrative 

purposes.

To begin, below in Table 3-18 are the values for the zero yaw drag areas (CdA (zero)) and 

WACdA at 55 miles per hour (mph) for each sleeper cab tractor and 53’ trailer, averaged across 

three OEM trailers with trailer side skirt #2 installed on the trailer. 

Table 3-18 Zero Yaw and Wind Average Drag Area RSWT Results for Tractor-Trailer Combinations with a 

Trailer Side Skirt (Results for Trailer Side Skirt #2 Shown).

TRACTOR CdA

(ZERO)

CdA DELTA 

VS. 

STANDARD 

TRAILER

CdA % 

DELTA VS. 

STANDARD 

TRAILER

WACdA

(@ 55 

MPH)

WACdA

DELTA VS. 

STANDARD 

TRAILER

WACdA % 

DELTA VS. 

STANDARD 

TRAILER

WACDA –

CDA

DELTA

(WACDA

OFFSET)

WACDA

– CDA

% 

DELTA 

(VS.

CDA)

Sleeper 1 4.914 0.511 9.42% 5.419 0.624 10.33% 0.505 10.27%

Sleeper 2 4.962 0.594 10.68% 5.705 0.700 10.94% 0.742 14.96%

Sleeper 3 5.056 0.566 10.07% 5.598 0.687 10.93% 0.542 10.73%

Sleeper 4 5.003 0.560 10.07% 5.627 0.714 11.26% 0.625 12.49%

Day 5.298 0.311 5.55% 5.755 0.612 9.61% 0.457 8.63%

Sleeper 

Average (2

out of 4 

tractors w/ 

matching 

coastdown 

data)

4.938 0.552 10.1% 5.562 0.662 10.6% 0.624 12.6%

Based on the results in Table 3-18, there is good alignment in the data (CdA standard 

deviation of 0.06, 1.21 percent relative standard error; WACdA standard deviation of 0.12, 2.16

percent relative standard deviation) across sleeper cab tractors and trailers equipped with trailer 

side skirts (e.g., a similar trend was demonstrated in the data for trailer side skirt #1 as well). 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-48

Similar to the method comparison in 

Figure 3-7 for zero degree drag area and a standard trailer, Figure 3-9 below shows the 

WACdA method comparison for three of the four Class 8, high roof, sleeper cab tractors with a 

trailer equipped with trailer side skirts consistent with the proposed HD GHG Phase 2 reference 

trailer modifications. The results are shown for constant speed testing and WACdA RSWT 

testing at 55mph assuming a 7mph wind speed, normalized to the coastdown test results. As 

discussed above, these results are for a specific set of tractor-trailer models, specifications and 

facilities for those tools and, thus, a similar or equivalent level of agreement is not guaranteed for 

using different criterion. However, as shown for HD GHG Phase 1, there is a certain level of 

agreement achievable using set conditions for a given tractor-trailer model as proposed under 

HD GHG Phase 2.

This is prior to any adjustment of the coastdown result to incorporate wind averaging 

which would theoretically increase the coastdown values upward since the wind average drag 

area is higher than zero degree yaw drag area. However, this is a useful comparison to 

demonstrate how the use of wind average drag in HD GHG Phase 1 may have influenced the 

level of agreement between the methods, and the resulting OEM Falt-aero values (e.g., Falt-aero 

values closer to 1), versus the use of zero yaw drag adopted in HD GHG Phase 1. We will 

explore the method comparison further with the inclusion of wind average drag later in this 

section.

1.00 1.00 1.00

1.03

0.95

0.97

0.92

0.90 0.93

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Sleeper 1 Sleeper 2 Sleeper 3

Coastdown

Constant Speed

RSWT

(Cd*A @ zero yaw)
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-49

Figure 3-9 Unadjusted Method Comparison for Coastdown, Constant Speed and RSWT (WACdA @ 55 mph

w/ 7 mph wind) Testing Normalized to the Coastdown Results, for Class 8 High Roof Sleeper Cab Tractors 

and Trailers Equipped with Trailer Side Skirts. Note: The results of Sleeper 1 and Sleeper 3 are not directly 

comparable since the test articles do not match but are shown to represent overall industry trend given a 

manufacturer’s unique fleet. 

1.00 1.00 1.00

0.96

0.98 1.00 1.02

1.02

1.00

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Sleeper 1 Sleeper 2 Sleeper 3

Coastdown

Constant Speed

RSWT (WACD*A @ 7/55)
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-50

Next, the results from the RSWT were used to develop a virtual WACdA value for 

coastdown testing. Specifically, the coastdown results and the WACdA to CdA delta will be used 

to correct the coastdown results. As previously mentioned, we could not match the exact tractor 

model for RSWT and on-road testing for one of the tractors. In addition, we have not completed 

on-road testing for another tractor model that has been tested in the RSWT. Therefore, for this 

exercise, we will use: the average of the RSWT results above for two out of the four sleeper cab 

tractors with trailer skirts shown in the last row of Table 3-18; and the average of coastdown 

results for two of the three tested sleeper cab tractors with trailer skirts (average: 5.549 [n=2]); to 

demonstrate the coastdown yaw sweep correction.

To begin the conversion from alternate method to coastdown, we first calculate certain 

parameters using the results from above: the wind average drag area – zero yaw drag area offset; 

and the Falt-aero using the coastdown and the zero yaw drag area results; shown in Table 3-19

below. 

Table 3-19 Converting Wind Average Drag Area from the Alternate Method to Virtual Wind Average Drag 

Area for the Coastdown Reference Method: Sample Calculations for Certified Configuration Calculations.

CERTIFIED CONFIGURATION USING COASTDOWN REFERENCE METHOD AND ALTERNATE 

METHOD

(SAMPLE DATA: RSWT RESULTS FOR DAY CAB IN TABLE 3-18)

Test Method/Source Variable Value

Coastdown (reference

method)

(CdA)zero,coastdown 5.5

Drag area from 

RSWT (or other alt. 

method)

(CdA)zero,wind 

tunnel

4.9

Wind average drag 

area RSWT (or other 

alt. method)

(CdA)wad wind 

tunnel

5.6

Alternate Method 

Factor

Falt-aero Falt-aero = (CdA)zero, coastdown / (CdA)zero,wind tunnel = 5.5/4.9 = 1.12 

Wind Average Offset WACd-CdA0

Offset

(CdA)wad,wind tunnel – (CdA)zero,wind tunnel = 5.6 – 4.9 = 0.7

CdAwad Calculations

(for bin determination 

and GEM input):

Wind Average 

Equivalent 

Coastdown

(CdA)wad

(CdA)wad = (CdA)zero, coastdown + ((CdA)wad,wind tunnel – (CdA)zero,wind tunnel)*Faltaero 

= 5.5+(5.6-4.9)*1.12 

= 5.5 + 0.7*1.12

(CdA)wad = 6.3

Using the Wind Average Equivalent Coastdown value of 6.2, a manufacturer would 

identify the appropriate bin for that value and use the associated aerodynamic GEM input for 

determining CO2 emissions and fuel consumption; which is discussed below. For now, the 

alternate method factor, Falt-aero, is not used, however, it will be important later for certifying 

additional configurations.

Once the certified configuration values have been derived, the alternate aerodynamic 

method can be used to certify additional configurations using the Falt-aero from the certified 

configuration. For this example, we will use the same data from Table 3-18 and from Table 3-19

above for the Day Cab to represent a completely different configuration since the data from the 

sleeper cab tractors is all very similar (Note: In reality, a manufacturer would not be allowed to 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-51

use the day cab for sleeper certification, and vice versa). Thus, this approach is limited to the 

example calculations for illustrative purposes). Table 3-20 shows the calculations using the data 

from Table 3-18 and Table 3-19 for the day cab. 

Specifically, Table 3-20 shows two cases: a case where the manufacturer has generated 

the wind average drag area from the alternate method and another case where the manufacturer 

does not have the wind average drag area but, instead, has the zero yaw drag area from the 

alternate method. In the latter case where the manufacturer does not have the wind average drag 

area, we are proposing that the manufacturer would use a default wind average drag area-zero 

yaw drag area of 0.80.

Table 3-20 Converting Wind Average Drag Area from the Alternate Method to Virtual Wind Average Drag 

Area for the Coastdown Reference Method: Sample Calculations for Additional Configurations Using RSWT 

Day Cab Data

ADDITIONAL CERTIFIED CONFIGURATIONS USING ALTERNATE METHOD

(SAMPLE DATA: RSWT RESULTS FOR DAY CAB IN TABLE 3-18)

Test Method/Source Variable Example: 

Previous Certified Configuration 

Data

Additional Configuration using 

Alternate Method

(Example: Day Cab Data)

Coastdown 

(reference method)

(CdA)zero,coastdown 5.5 Not Available; 

alternate method used

Alt. method Drag 

area

(CdA)zero,alt method 4.9 5.3

Alt. method wind 

average drag area

(CdA)wad,alt method 5.6 5.8

Calculated Using 

Certified 

Configuration in 

Table 3-19

Alternate Method 

Factor (Falt-aero)

Falt-aero = (CdA)zero,coastdown / (CdA)zero,wind tunnel = 5.5/4.9 = 1.12

(from Table 3-19 for certified case)

Actual Wind 

Average Offset

WACd-CdA0 

Offset

0.7 0.5

Default Wind 

Average Offset

Default Offset 0.8 0.8

CdAwad Calculations

(for bin 

determination and 

GEM input):

With (CdA)wad, alt 

method

(CdA)wad = (CdA)wad,alt method * Faltaero

= 5.6 * 1.12 

= 6.3

(CdA)wad = (CdA)wad,alt method * 

Falt-aero

= 5.8 * 1.12 

= 6.5

Without (CdA)wad, 

alt method; Instead, 

use (CdA)zero, 

coastdown (if 

applicable) or 

(CdA)zero, alt method

with a default 

offset value of 

0.80

(CdA)wad = (CdA)zero,coastdown + 

default offset 

= 5.5 + 0.80

= 6.3

Not Applicable; 

Only alternate method is used

(CdA)wad = (CdA)zero,alt method * Faltaero + default offset

= 4.9 * 1.12 + 0.80

= 6.3

(CdA)wad = (CdA)zero, alt method * 

Falt-aero + default offset

= 5.3 * 1.12 + 0.80

= 6.7

In contrast to Falt-aero that is calculated once for the certified configuration and may be 

applied to additional configurations, the wind average drag area – drag area (WACdA – CdA) 

offset is configuration dependent, and must be calculated for and used solely for that 

configuration. It may be possible to use a single wind average – drag area offset for grouping 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-52

similar configurations but this would need to be supported by data showing the level of variation 

between the configurations to justify such a grouping, and approved by the agencies prior to use. 

Now that we have applied the adjustment for wind average drag area to the coastdown, 

we will revisit the method comparison using this approach. Below in Figure 3-10 is the data 

from Figure 3-9 with the values for coastdown adjusted to account for wind average drag. In 

addition, the constant speed test results have been adjusted by the same factor to adjust for wind 

average and maintain its relative agreement with the coastdown test results. As shown in Figure 

3-10, the results for the RSWT are lower than the coastdown test results, as expected. Further, of 

note is the fact that if we analyze the method agreement for the zero yaw RSWT test results for a 

trailer with a skirt, prior to wind average adjustment and normalized to the coastdown test 

results, you would get results of 0.91 for Sleeper 1, 0.89 for Sleeper 2, and 0.90 for Sleeper 3. 

These are the results shown in Figure 3-10, demonstrating that the equations above are 

adequately accounting for wind average drag area in the coastdown results (e.g., the relative 

difference between the adjusted coastdown and RSWT wind average drag area is the same as the 

relative difference between the original coastdown result and the zero yaw drag area).
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-53

Figure 3-9 Adjusted Method Comparison for Coastdown, Constant Speed and RSWT (WACdA @ 55 mph 

w/ 7 mph wind) Testing Normalized to the Coastdown Results with wind average adjustment applied, for 

Class 8 High Roof Sleeper Cab Tractors and Trailers Equipped with Trailer Side Skirts. Note: The results of 

Sleeper 1 and Sleeper 3 are not directly comparable since the test articles do not match but are shown to 

represent overall industry trend given a manufacturer’s unique fleet. 

1.00 1.00 1.00

0.96

0.98 1.02

0.91

0.89 0.90

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Sleeper 1 Sleeper 2 Sleeper 3

Coastdown

Constant Speed

RSWT

(Cd*A @ zero yaw)
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-54

Finally, below in Table 3-21 are the wind average equivalent coastdown values to 

illustrate how today’s tractors fit into the proposed HD Phase 2 aerodynamic standards. These 

values were developed using the coastdown results for each tractor and trailer equipped with a 

trailer side skirt, and applying the principles above with the corresponding zero yaw drag area 

and wind-average drag area from the RSWT. The corresponding aerodynamic bins and GEM 

input are provided as well based on the proposed bin values in Section III.E.(2)(a)(iv) of the 

preamble. A manufacturer would use the resulting aerodynamic GEM input to generate the 

configuration-specific CO2 emission and fuel consumption value for certification. As shown 

below, most tractors today would qualify for Bin III with varying degrees of opportunity to move 

into improved bins.

Table 3-21 Wind Average Equivalent Coastdown Values Used to Develop the Proposed HD Phase 2 

Aerodynamic Standards and Corresponding Aero Bin and Aero GEM Input.

TRACTOR CONFIG CSTDWN CdA0 WACd Falt-aero WACd

OFFSE

T

WACd X

Falt-aero

CSTDWN

+ WACd

OFFSET

HD PHASE 

2 

PROPOSED 

BIN #

BIN III 

AERO TEST 

RESULT

BIN III 

AERO GEM 

INPUT

Sleeper 1 skirt 5.4 4.91 5.42 1.11 0.505 6.0 6.0 Bin III 6.0-6.5 6.3

Sleeper 2 skirt 5.6 4.96 5.70 1.15 0.742 6.5 6.4 Bin III 6.0-6.5 6.3

Sleeper 3 skirt 5.6 5.06 5.60 1.11 0.542 6.2 6.2 Bin III 6.0-6.5 6.3

This table is only intended to illustrate how the tractors would fit into the proposed bin 

structure but may not be completely appropriate for bin setting. Specifically, in two cases, there 

were tractors where either the model designation or model year did not match between the onroad and RSWT testing. As a result, it is better to look at the data in aggregate rather than on an 

individual vehicle basis.

Using the average of the coastdown drag areas (avg = 5.5), the average of the wind 

average offsets (avg = 0.6), and the alternate method factor (Falt-aero) of 1.15 from the tractor 

(Sleeper 2) with matching vehicles between on-road and wind tunnel testing; in the equation in 

Table 3-18 for a certified configuration (e.g., CdAzero, coastdown + (CdAwad, wind tunnel – CdAzero, wind tunnel) * Faltaero) you get a CdAwad value of 6.23. This value is almost exactly in the middle of the proposed 

Bin III standards and, therefore, we think that the proposed standards are appropriate. Further, 

the data in the Table 3-21 above provides some insight into the variability and range of high roof 

sleeper cab tractors from across the manufacturers that might be expected to be certified for HD

Phase 2. Thus, although we do not have a complete set of matching datasets in all cases, we 

believe the values and bandwidth are appropriate for the HD Phase 2 proposed bin standards

based on the data and calculations above.

3.2.10 Delta Drag Area Results to Support Proposed HD Phase 2 Trailer 

Regulations

Similar to the tractor aerodynamic assessment in Section 3.2.9 above, we can use the data 

generated during our aerodynamic assessment test programs to support development of trailer 

aerodynamic standards. As previously discussed in Section 3.2.5, the proposed trailer 

regulations would use the delta CdA from any of the accepted aerodynamic methods. Table 3-22

below shows the delta CdAs across tractors, trailers, trailer configurations and aerodynamic test 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-55

methods using data from Table 3-8, Table 3-9 and Table 3-10. The corresponding trailer 

certification bin and trailer aerodynamic GEM inputs are provided as well based on the proposed 

values in Section IV of the preamble, IV.F.(3)(b)(iv), Table IV-31.

Table 3-22 Delta CDAs from Various Tractors, Trailers, Trailer Configurations and Aerodynamic Test 

Methods with Corresponding Trailer Aerodynamic Bins

CAB TYPE CONFIGURATION DELTA CdA

(VS. STANDARD)

TRAILER CERT. BIN TRAILER AERO 

GEM INPUT

Coastdown Testing

Sleeper 1 With Trailer Skirt #2 0.5 III 0.5

With Trailer Skirt and Boat Tail 0.9 IV 0.9

Sleeper 2 With Trailer Skirt #2 0.6 III 0.5

With Trailer Skirt and Boat Tail 1.1 IV 0.9

Sleeper 3 With Trailer Skirt #2 0.5 III 0.5

With Trailer Skirt and Boat Tail 0.7 IV 0.9

Constant Speed Testing

Sleeper 1 With Trailer Skirt #2 0.7 IV 0.9

With Trailer Skirt and Boat Tail 1.1 IV 0.9

Sleeper 2 With Trailer Skirt #2 0.5 III 0.5

Reduced Scale Wind Tunnel Testing (RSWT)

Sleeper 1 With Trailer Skirt #1 0.423 III 0.5

With Trailer Skirt #2 0.511 III 0.5

With Boat Tail 0.340 III 0.5

With Trailer Skirt #2 and Boat Tail 0.972 IV 0.9

Sleeper 2 With Trailer Skirt #1 0.586 III 0.5

With Trailer Skirt #2 0.594 III 0.5

With Boat Tail 0.365 III 0.5

With Trailer Skirt #2 and Boat Tail 1.092 IV 0.9

Sleeper 3 With Trailer Skirt #1 0.477 III 0.5

With Trailer Skirt #2 0.566 III 0.5

With Boat Tail 0.318 III 0.5

With Trailer Skirt #2 and Boat Tail 1.056 IV 0.9

Sleeper 4 With Trailer Skirt #1 0.481 III 0.5

With Trailer Skirt #2 0.557 III 0.5

With Boat Tail 0.332 III 0.5

With Trailer Skirt #2 and Boat Tail 1.121 IV 0.9

Day With Trailer Skirt #1 0.256 II 0.2

With Trailer Skirt #2 0.311 III 0.5

With Boat Tail 0.370 III 0.5

With Trailer Skirt #2 and Boat Tail 0.699 III 0.5

In general, Table 3-22 shows that individual components (e.g., trailer side skirts, boat 

tails) added to a trailer might qualify for Bin III while combinations of devices (e.g., trailer side 

skirt and boat tail) added to a trailer might qualify for Bin IV. For the day cab, a basic skirt such 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-56

as Skirt #1 would qualify for Bin II as opposed to an advance skirt such as Skirt #2 which would 

qualify for Bin III.

Trailer manufacturers would follow a similar process of using the delta CdA values for 

their trailer devices or improved trailer designs to identify the appropriate trailer certification bin 

and the trailer aero GEM input for that bin. Accordingly, these values were used to develop the 

trailer certification bins and aero GEM inputs to support the trailer regulations in HD Phase 2.

3.3 Tire Rolling Resistance

The agencies are proposing that the ISO 28580 test method be used to determine rolling 

resistance and the coefficient of rolling resistance. A copy of the test method can be obtained 

through the American National Standards Institute.13

3.3.1 Reason for Using ISO 28580

EPA’s SmartWay Partnership Program started to identify equipment and feature 

requirements for SmartWay-designated Class 8 over-the-road tractors and trailers in 2006. In 

order to develop a tire rolling resistance specification for SmartWay-designated commercial 

trucks, EPA researched different test methods used to evaluate tire rolling resistance, reviewing 

data and information from tire manufacturers, testing laboratories, the State of California, the 

Department of Transportation, tractor manufacturers, and various technical organizations. After 

assessing this information, EPA determined that its SmartWay program would use the SAE 

J126914 tire rolling resistance method until the ISO 2858015 method (at that time under 

development) was finalized, at which time the Agency would consider moving to this method for 

its SmartWay program.

During this same time period, the National Highway Traffic Safety Administration 

(NHTSA) conducted an evaluation of passenger vehicle tire rolling resistance test methods and 

their variability.

16

 Five different laboratory test methods at two separate labs were evaluated. 

The NHTSA study focused on passenger tires; however, three of the four test methods evaluated 

can be used for medium-duty and heavy-duty tractor tires. The methods evaluated were SAE 

J1269, SAE J245217 (not applicable for medium-duty or heavy-duty tractor tires), ISO 1816418

and ISO 28580. The NHTSA study showed significant lab to lab variability between the labs 

used. The variability was not consistent between tests or types of tire within the same test. The 

study concluded that a method to account for this variability is necessary if the rolling resistance 

value of tires is to be compared (NHTSA, 2009). Because of laboratory variability, NHTSA 

recommended that the use of ISO 28580 is preferred over the other test methods referenced.

ISO 28580 is preferred because the test method involves laboratory alignment between a 

“reference laboratory” and “candidate laboratory.” The ISO technical committee involved in 

developing this test method also has the responsibility for determining the laboratory that will 

serve as the reference laboratory. The reference laboratory would make available an alignment 

tire that can be purchased by candidate laboratories. The candidate laboratory would identify its 

reference machine. However, at this time, the reference laboratory and alignment tires have not 

been identified.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-57

3.3.2 Measurement Method and Results

The ISO 28580 test method includes a specific methodology for “light truck, commercial 

truck and bus” tires, and it has 4 measurement methods, force, torque, deceleration, and power, 

all of which appear to be suitable for use.

The results of the ISO 28580 test are intended for use in vehicle simulation modeling, 

such as the model used to assess the effects of various technology options for national 

greenhouse gas and fuel economy requirements for commercial trucks (see Chapter 4). The 

results are usually expressed as a rolling resistance coefficient and measured as kilogram per 

metric ton (kg/metric ton) or as dimensionless units (1 kg/metric ton is the same as the 

dimensionless unit 0.001). The results are corrected for ambient temperature drum surface and 

drum diameter as specified in the proposed test method.

3.3.3 Sample Size

The rolling resistance of tires within the same model and construction are expected to be 

relatively uniform. In the study conducted by NHTSA, only one individual tire had a rolling 

resistance value that was significantly different from the other tires of the same model. The 

effect of production variability can be further reduced by conducting three replicate tests and 

using the average as the value for the rolling resistance coefficient. Tire models available in 

multiple diameters may have different values of rolling resistance for each diameter because 

larger diameter tires can produce lower rolling resistance than smaller diameters under the same 

load and inflation conditions. If the size range within a tire model becomes large enough that a 

given tire size is no longer “substantially similar” in rolling resistance performance to all other 

tire sizes of that model, then good engineering judgment should be exercised as to whether the 

differently-sized tire shall be treated, for testing and vehicle simulation purposes, as a distinct tire 

model. For Class 8 tractors that typically use tires that fit on 22.5” or 24.5” wheels, this situation 

might occur with 17.5” tires, more commonly used on moving vans and other applications that 

require a low floor.

3.3.4 Tire Size

In Phase 2, the agencies propose to require manufacturers to enter the tire loaded radius 

as a GEM input. While this rulemaking does not include tire size among the technologies 

applied to improve fuel efficiency, this measurement is among the driveline parameters 

necessary for GEM to calculate a vehicle speed for a given engine speed. Because there is a 

wide range of possible measurements for loaded radius, the agencies are specifying a proposed 

measurement procedure. Tire sizes can be measured using an overall diameter or a static loaded 

radius. Deflection is typically between 24 and 33 percent depending on the tire design. In the 

first 100-200 miles of a tire’s useful life, there will be a break-in process during which a 

commercial tire can “grow” one to two percent, up to 18 mm. Because this growth affects the air 

pressure in the tire, it’s important to specify the air pressure under which the loaded radius 

measurement is performed. The Society of Automotive Engineers (SAE) has published 

recommended practice J1025 for determining the revolutions per mile of new truck tires.19 

Consistent with that recommended practice, the agencies propose that manufacturers would 

quantify the loaded tire radius of the drive tire, NIST traceable within ±0.5 percent uncertainty, 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-58

by measuring the perpendicular distance from the axis of rotation of the loaded tire to the surface 

on which it is rolling. Load the tire to 85 percent of the maximum load capacity specified by the 

manufacturer, at the corresponding air inflation level. See 40 CFR 1037.520(j).

3.4 Duty Cycle 

Certification duty cycles have a significant impact on the GHG emissions from a truck 

and how technologies are assessed. Every truck has a different duty cycle in-use. Therefore, it is 

very challenging to develop a uniform duty cycle which accurately assesses GHG improvements 

and fuel efficiency from technologies relative to their performance in the real world. 

The duty cycle attributes that impact a vehicle’s performance include average speed, 

maximum speed, acceleration rates, deceleration rates, number of stops, road grade, power takeoff operation, and idling time. Average and maximum speeds are the attributes which have the 

greatest impact on aerodynamic technologies. Vehicle speed also impacts the effect of low 

rolling resistance tires. The effectiveness of extended idle reduction measures is determined by 

the amount of time spent idling. Lastly, hybrid technologies demonstrate the greatest 

improvement on cycles which include a significant amount of stop-and-go driving due to the 

opportunities to recover braking energy. In addition, the amount of power take-off operation will 

impact the effectiveness of some vocational hybrid applications.

The ideal duty cycle for a line-haul truck would account for a significant amount of time 

spent cruising at high speeds. A pickup and delivery truck duty cycle would contain a 

combination of urban driving, some number of stops, and limited highway driving. If the 

agencies propose an ill-suited duty cycle for a regulatory subcategory, it may drive technologies 

where they may not see the in-use benefits. For example, requiring all trucks to use a constant 

speed highway duty cycle would drive significant aerodynamic improvements. However, in the 

real world a pickup and delivery truck may spend too little time on the highway to realize the 

benefits of aerodynamic enhancements. In addition, the extra weight of the aerodynamic fairings 

would actually penalize the GHG performance of that truck in urban driving and may reduce its 

freight carrying capability. 

3.4.1 Duty Cycles Considered

In HD Phase 1, the agencies selected three duty cycles for certification testing: the 

Transient portion of the California Air Resource Board (CARB) Heavy Heavy-Duty Truck 5 

Mode Cycle, 55 mph cruise (without grade), and 65 mph cruise (without grade).

For HD Phase 2, the agencies carefully considered which duty cycles are appropriate for 

the different regulatory subcategories. We considered several duty cycles in the development of 

the rulemaking including EPA’s MOVES model; the Light-Duty FTP75 and HFET; Heavy-Duty 

UDDS; World Wide Transient Vehicle Cycle (WTVC); Highway Line Haul; Hybrid Truck User 

Forum (HTUF) cycles; and California CARB’s Heavy-Heavy-Duty Truck 5 Mode Cycle.

MOVES Medium-Duty and Heavy-Duty schedules were developed based on three 

studies. Eastern Research Group (ERG) instrumented 150 medium and heavy-duty vehicles, 

Battelle instrumented 120 vehicles instrumented with GPS, and Faucett instrumented 30 trucks 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-59

to characterize their in-use operation.20 ERG then segregated the driving into freeway and nonfreeway driving for medium and heavy-duty vehicles, and then further stratified vehicle trips 

according the predefined ranges of average speed covering the range of vehicle operation. 

Driving schedules were then developed for each speed bin by creating combinations of idle-toidle “microtrips” until the representative target metrics were achieved. The schedules developed 

by ERG are not contiguous schedules which would be run on a chassis dynamometer, but are 

made up of non-contiguous “snippets” of driving meant to represent target distributions. This 

gives MOVES the versatility to handle smaller scale inventories, such as intersections or sections 

of interstate highway, independently. 

The FTP75 and HFET duty cycles are used extensively for Light-Duty emissions and 

CAFE programs. Our assessment is that these cycles are not appropriate for HD trucks for two 

primary reasons. First, the FTP has 24 accelerations during the cycle which are too steep for a 

Class 8 combination tractor to follow. Second, the maximum speed is 60 mph during the 

HWFEC, while the national average truck highway speed is 65 mph. 

The Heavy-Duty Urban Dynamometer Driving Cycle was developed to determine the 

Heavy-Duty Engine FTP cycle. The cycle was developed from CAPE-21 survey data which 

included information from 44 trucks and 3 buses in Los Angeles and 44 trucks and 4 buses in 

New York in 1977. The cycle was computer generated and weighted to represent New York 

non-freeway (254 sec), Los Angeles non-freeway (285 sec), Los Angeles freeway (267 sec), 

New York non-freeway (254 sec) to produce a nearly 50/50 weighting of highway cruise and 

urban transient. We believe this cycle is not appropriate for our program for several reasons. 

The maximum speed on the UDDS is 58 mph which is low relative to the truck speed limits in 

effect today. The 50/50 weighting of cruise to transient is too low for combination tractors and 

too high for vocational vehicles and the single cycle does not provide flexibility to change the 

weightings. Lastly, the acceleration rates are low for today’s higher power trucks.

The World Harmonized WTVC was developed by the UN ECE GRPE group. It 

represents urban, rural, and motorway operation. The cycle was developed based on data from 

20 straight trucks, 18 combination tractors, and 11 buses total from Australia, Europe, Japan, and 

the US. EPA has a desire to harmonize internationally, however, we believe that this single 

cycle does not optimally cover the different types of truck operation in the United States and 

does not provide the flexibility to vary the weightings of a single cycle.

The Highway Line Haul schedule was created by Southwest Research Institute, using 

input from a group of stakeholders, including EPA, Northeastern States for Coordinated Air Use 

Management (NESCAUM), several truck and engine manufacturers, state organizations, and 

others, for a NESCAUM heavy truck fuel efficiency modeling and simulation project. The cycle 

is 103 miles long and incorporates grade and altitude. This cycle is a good representation of line 

haul operation. However, the altitude changes cannot be incorporated into a chassis 

dynamometer or track test and the cycle is also too long for a typical chassis dynamometer test. 

The Calstart-Weststart Hybrid Truck Users Forum is developing cycles to match the 

characteristics of truck applications which are expected to be first to market for hybrids. The 

cycles include the Manhattan Bus Cycle, Orange County Bus Cycle, Class 4 Parcel Delivery, 

Class 6 Parcel Delivery, Combined International Local and Commuter Cycle (CILCC), 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-60

Neighborhood Refuse, Utility Service, and Intermodal Drayage cycles. The cycles are very

application-specific and appropriately evaluate each vocation. However, the use of these types 

of application specific cycles in a regulatory scheme would lead to a proliferation of cycles for 

every application, an outcome that is not desirable.

The CARB 5 Mode cycle was developed by California CARB from heavy-duty truck 

data gathered from 1997 through 2000.21 Data was collected from real world driving from 

randomly selected vehicles. The data was gathered from 140 heavy-duty trucks by Battelle and 

from 31 heavy-duty trucks in a study conducted by Jack Faucett and Associates. The final data 

set included 84 of these heavy duty trucks covering over 60,000 miles and 1,600 hours of 

activity. The cycles were developed to reflect typical in-use behavior as demonstrated from the 

data collected. The four modes (idle, creep, transient, and cruise) were determined as distinct 

operating patterns, which then led to the four drive schedules. The cycle is well accepted in the 

heavy-duty industry. It was used in the CRC E55/59 Study which is the largest HD chassis 

dynamometer study to date and used in MOVES and EMFAC to determine emission rate inputs; 

EPA’s biodiesel study which used engine dynamometer schedules created from CARB cruise 

cycle; the HEI ACES Study: WVU developed engine cycles from CARB 4-mode chassis cycles; 

CE/CERT test; and by WVU to predict fuel efficiency performance on any duty cycle from 

CARB 5 mode results. The modal approach to the cycles provides flexibility in cycle weightings 

to accommodate a variety of truck applications. A downside of the cycle is that it was developed 

from truck activity in California only.

3.4.2 Proposed Duty Cycles

3.4.2.1 Highway Cruise Cycles

The agencies analyzed the average truck speed limit on interstates and other freeways to 

identify the appropriate speed of the highway cruise cycles. State speed limits for trucks vary

between 55 and 75 mph, depending on the state.22 The median urban and rural interstate speed 

limit of all states is 65 mph. The agencies also analyzed the speed limits in terms of VMTweighting. The agencies used the Federal Highway Administration data on Annual Vehicle 

Miles for 2008 published in November 2009 to establish the vehicle miles travelled on rural and 

urban interstates broken down by state. The VMT-weighted national average speed limit is 63 

mph based on the information provided in Table 3-23. The results of this analysis led to the 

adoption of the High Speed (65 mph) and Low Speed (55 mph) Cruise duty cycles in Phase 1.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-61

Table 3-23 VMT-Weighted National Truck Speed Limit 

STATE RURAL 

INTERSTATE 

SPEED LIMIT

URBAN 

INTERSTATE 

SPEED LIMIT

RURAL 

INTERSTATE 

MILES

URBAN 

INTERSTATE 

AND OTHER 

FREEWAYS 

MILES

U.S. 

WEIGHTED 

VMT 

FRACTION 

RURAL

U.S. 

WEIGHTED 

VMT 

FRACTION 

URBAN

VMT 

WEIGHTED 

SPEED 

LIMIT

AL 70 65 5,643 7,950 0.6% 0.8% 0.968

AK 55 55 803 662 0.1% 0.1% 0.086

AZ 75 65 6,966 13,324 0.7% 1.4% 1.474

AR 65 55 4,510 4,794 0.5% 0.5% 0.591

CA 55 55 17,681 123,482 1.9% 13.1% 8.242

CO 75 65 4,409 11,745 0.5% 1.2% 1.161

CN 65 55 715 13,485 0.1% 1.4% 0.837

DE 55 55 - 1,694 0.0% 0.2% 0.099

DC 55 55 - 813 0.0% 0.1% 0.047

FL 70 65 9,591 37,185 1.0% 3.9% 3.279

GA 70 55 9,433 21,522 1.0% 2.3% 1.958

HA 60 60 110 2,403 0.0% 0.3% 0.160

ID 65 65 2,101 1,250 0.2% 0.1% 0.231

IL 65 55 8,972 23,584 1.0% 2.5% 1.996

IN 65 55 7,140 10,850 0.8% 1.2% 1.126

IA 70 55 4,628 2,538 0.5% 0.3% 0.492

KA 75 75 3,242 5,480 0.3% 0.6% 0.694

KE 65 65 6,566 6,834 0.7% 0.7% 0.925

LA 70 70 5,489 7,708 0.6% 0.8% 0.981

ME 65 65 2,207 958 0.2% 0.1% 0.218

MA 65 65 3,484 18,792 0.4% 2.0% 1.537

MS 70 70 1,257 20,579 0.1% 2.2% 1.623

MI 60 60 5,245 20,931 0.6% 2.2% 1.667

MN 70 60 4,150 12,071 0.4% 1.3% 1.077

MS 70 70 4,103 4,004 0.4% 0.4% 0.602

MO 70 60 5,972 16,957 0.6% 1.8% 1.524

MT 65 65 2,350 343 0.2% 0.0% 0.186

NE 75 65 2,590 1,653 0.3% 0.2% 0.320

NV 75 65 1,826 5,286 0.2% 0.6% 0.510

NH 65 65 1,235 2,574 0.1% 0.3% 0.263

NJ 65 55 1,609 25,330 0.2% 2.7% 1.590

NM 75 65 4,530 2,667 0.5% 0.3% 0.545

NY 65 55 6,176 37,306 0.7% 4.0% 2.604

NC 70 70 5,957 19,216 0.6% 2.0% 1.871

ND 75 75 1,394 374 0.1% 0.0% 0.141
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-62

OH 65 65 9,039 27,830 1.0% 3.0% 2.544

OK 75 70 5,029 7,223 0.5% 0.8% 0.937

OR 55 55 4,109 5,734 0.4% 0.6% 0.575

PA 65 55 10,864 21,756 1.2% 2.3% 2.020

RI 65 55 404 2,948 0.0% 0.3% 0.200

SC 70 70 7,355 6,879 0.8% 0.7% 1.058

SD 75 75 1,960 648 0.2% 0.1% 0.208

TN 70 70 8,686 13,414 0.9% 1.4% 1.642

TX 70 70 15,397 71,820 1.6% 7.6% 6.481

UT 75 65 3,117 6,165 0.3% 0.7% 0.674

VT 65 55 1,216 443 0.1% 0.0% 0.110

VA 70 70 8,764 18,907 0.9% 2.0% 2.056

WA 60 60 4,392 15,816 0.5% 1.7% 1.287

WV 70 65 3,195 3,175 0.3% 0.3% 0.456

WI 65 65 5,197 9,139 0.6% 1.0% 0.989

WY 75 75 2,482 474 0.3% 0.1% 0.235

In establishing the highway cruise cycles in Phase 1, we realized that we did not address 

the effect of road grade on emissions. Therefore, for Phase 2, we are proposing to alter the High 

Speed Cruise and Low Speed Cruise modes to reflect road grade for the constant speed cycles at 

65 mph and 55 mph respectively. Based on input from trucking fleets and truck manufacturers, 

we believe this is representative of in-use operation, wherein truck drivers use cruise control 

whenever possible during periods of sustained higher speed driving and road grade varies.

To this end, the U.S. Department of Energy and EPA have partnered to support a project 

aimed at evaluating, refining and/or developing the appropriate road grade profiles for the duty 

cycles that would be used in the certification of heavy-duty vehicles to the GHG emission and 

fuel efficiency Phase 2 standards. The National Renewable Energy Laboratory (NREL) is 

leading a project which will refine the existing highway cruise duty cycles. In the course of this 

work, NREL has developed several activity-weighted road grade profiles which are

representative of U.S. limited-access highways using high-accuracy road grade and countyspecific data for vehicle miles traveled. Either a single road grade profile representative of the 

nation’s limited-access highways will be chosen for use in the highway cruise cycles or two 

activity-weighted grade profiles will be selected if analysis demonstrates that they should be 

different for speed limits of 55 and 65 mph. The profiles are distance-based and cover a 

maximum distance of 15 miles. In addition to NREL work, the agencies have independently 

developed another candidate road grade profile for use in the 55 mph and 65 mph highway cruise 

cycles. While based on the same road grade database generated by NREL for U.S. restrictedaccess highways, its design is predicated on a different approach. This analysis of road grade 

profile options was not completed in time for use in developing the primary proposal. Therefore, 

for the proposal, the agencies selected an interim road grade profile for development of the 

proposed standards, which is described in Section III.E of the preamble to these rules. Based on 

preliminary results, it appears the interim road grade profile closely matches a national road 

grade profile on an absolute basis, before VMT weighting. The report documenting the NREL’s 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-63

and the agencies’ road grade work is available to the public in the docket, as is the agencies’ 

analysis of possible alternative vehicle standards developed using alternative road grade profiles.

3.4.2.2 Transient Cycle

The Phase 1 rule requires use of the Transient portion of the CARB’s Heavy Heavy-Duty

Truck 5 Mode Cycle. The agencies have found that this cycle reasonably represents transient 

operation of many heavy-duty vehicles, though it is a very short test cycle - less than 3 miles –

and can be driven in roughly 11 minutes. We are not proposing any changes to that cycle, and 

would continue to use it when certifying vehicles to the HD Phase 2 standards.

The agencies would like to note that we have also launched a project at the National 

Renewable Energy Laboratory (NREL) to determine the extent to which the Transient mode of 

the CARB Heavy-Duty Truck 5 Mode Cycle is representative of transient operation of Class 2b8 vocational vehicles. This analysis is being performed using NREL’s extensive vehicle activity

database and a variety of metrics such as average driving speed, kinetic intensity, idle time, 

maximum driving speed and standard deviation of speed. Should NREL recommend, and the 

agencies agree, that any subcategory of vocational vehicles is poorly represented by the 

Transient mode of the CARB cycle, a more representative transient test cycle will be adopted, 

possibly selected from test cycles already in use. This analysis was not completed in time for use 

in developing the primary proposal. The report documenting this work is available to the public 

in the docket, as is the agencies’ analysis of possible alternative vocational vehicle standards 

developed using an alternative transient duty cycle.

3.4.2.3 Idle Cycle

We are also proposing the addition of an idle-only cycle to determine both fuel 

consumption and CO2 emissions when a vehicle is idling, and recognize technologies that either 

reduce the fuel consumption rate or shut the engine off (and restart) during short-term idle events 

during the workday. The agencies are not expecting that this cycle would recognize technologies 

that allow the main engine to remain off during stationary vehicle operation with a PTO engaged 

and performing work. Those technologies would be recognized over the Hybrid-PTO test 

procedure defined in 40 CFR 1037.525. In this proposed idle-only cycle, based on user inputs, 

GEM would calculate CO2 emissions and fuel consumption at both zero torque (neutral idle) and 

with torque set to Curb-Idle Transmission Torque (as defined in 40 CFR 1065.510(f)(4) for 

variable speed engines) for use in the CO2 emission calculation in 40 CFR 1037.510(b). We are 

also proposing that GEM would calculate reduced CO2 and fueling for stop-start systems, based 

on an assumption that the effectiveness would represent a 90 percent reduction of the emissions 

that would occur if the vehicle had operated at Curb-Idle Transmission Torque over this cycle. 

This cycle is proposed to be applicable only for vocational vehicles using either the Regional, 

Multi-Purpose, or Urban composite duty cycles.

3.4.3 Weightings of Each Cycle per Regulatory Subcategory

Table 3-24 presents the Phase 1 final GEM duty cycle composite weightings for 

vocational vehicles and tractors.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-64

Table 3-24 Phase 1 Vehicle Duty Cycle Composite Weightings

VEHICLE 

CATEGORY

PHASE 1 COMPOSITE WEIGHTINGS OF DUTY 

CYCLE MODE

Transient 55 mph Cruise 65 mph Cruise

Vocational 42% 21% 37%

Vocational Hybrid 

Vehicles

75% 9% 16%

Day Cabs 19% 17% 64%

Sleeper Cabs 5% 9% 86%

3.4.3.1 Vocational Vehicles

In order to properly weight the “idle” time of each vehicle class and category 

independently of the idle time in the duty cycles, EPA is proposing that idle emissions are 

weighted with the driving cycles. In the HD Phase 1 rule the duty cycles were weighted by 

distance to properly reflect the vehicle miles traveled by each category. To incorporate “idle” 

emissions, the equation had to be modified to allow for the “idle” emissions to be time weighted 

with the driving cycles. The result of this is that the weighting factors for the driving cycles will 

still add up to 100 percent while the idle weighting factor will be less than 100 percent, reflecting 

the actual idle time of the vehicles by category. The agencies are proposing to modify the 

equation in 40 CFR 1037.510(b) to accommodate both the distance (non-idle) and time based 

(idle) weighting factors.

The proposed duty cycle weightings for each vocational vehicle test cycle are included in

Table 3-25.

Table 3-25 Proposed Phase 2 Duty Cycle Mode Composite Weightings

VEHICLE 

CATEGORY

DUTY CYCLE MODE

Transient 55 mph Cruise 65 mph Cruise Idle

Vocational 

Regional

50% 28% 22% 10%

Vocational MultiPurpose82% 15% 3% 15%

Vocational Urban 94% 6% 0% 20%

3.5 Tare Weights and Payload

We propose to continue defining the total weight of a truck as the combination of the 

truck’s tare weight, a trailer’s tare weight (if applicable), and the payload; as it was defined in the 

HD Phase 1 rule. The total weight of a truck is important because it in part determines the 

impact of technologies, such as rolling resistance, on GHG emissions and fuel consumption. As 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-65

the HD program is designed, it is important that the agencies define weights which are 

representative of the fleet while recognizing that the final weights are not representative of a 

specific vehicle. The sections below describe the agencies’ approach to defining each of these 

weights.

3.5.1 Truck Tare Weights

The tare weight of a truck will vary depending on many factors, including the choices 

made by the manufacturer in designing the truck (such as the use of lightweight materials, the 

cab configuration (such as day or sleeper cab), whether it has aerodynamic fairing (such as a roof 

fairing), and the specific options on the truck. 

The Class 8 combination tractor tare weights were developed based on the weights of 

actual tractors tested in EPA’s coastdown program. The empty weight of the Class 8 sleeper 

cabs with a high roof tested ranged between 19,000 and 20,260 pounds. The empty weight of the 

Class 8 day cab with a high roof tested was 17,840 pounds. The agencies derived the tare weight 

of the Class 7 day cabs based on the guidance of truck manufacturers. The agencies then 

assumed that a roof fairing weighs approximately 500 pounds. Based on this, the agencies are 

proposing the tractor tare weights as shown in Table 3-26.

Table 3-26 Tractor Tare Weights

MODEL 

TYPE

CLASS 8 CLASS 8 CLASS 8 CLASS 8 CLASS 8 CLASS 7 CLASS 7

Regulatory

Subcategory

Sleeper 

Cab High 

Roof

Sleeper 

Cab Mid 

Roof

Sleeper 

Cab Low 

Roof

Day Cab 

High 

Roof

Day Cab 

Low 

Roof

Day Cab 

High 

Roof

Day Cab 

Low 

Roof

Tractor Tare 

Weight (lbs)

19,000 18,750 18,500 17,500 17,000 11,500 11,000

The agencies developed the empty tare weights of the vocational vehicles based on the 

EDF report23 on GHG management for Medium-Duty Fleets. The EDF report found that the 

average tare weight of a Class 4 truck is 10,343 pounds, of a Class 6 truck is 13,942 pounds, and 

a Class 8 truck is 23,525 pounds. The agencies are proposing to continue to use the following 

tare weights:

 Light Heavy (Class 2b-5) = 10,300 pounds

 Medium Heavy (Class 6-7) = 13,950 pounds

 Heavy Heavy (Class 8) = 23,500 pounds

3.5.2 Trailer Tare Weights

We propose to continue to define the trailer tare weights used in the tractor program

based on measurements conducted during EPA’s coastdown testing and information gathered by 

ICF in the cost report to EPA, as adopted in the HD Phase 1 rule.24 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-66

A typical 53 foot box (or van) trailer has an empty weight ranging between 13,500 and 

14,000 pounds per ICF’s findings. The box trailer tested by EPA in the coastdown testing 

weighed 13,660 pounds. Therefore, the agencies are defining the empty box trailer weight as 

13,500 pounds.

A typical flatbed trailer weighs between 9,760 and 10,760 per the survey conducted by

ICF. EPA’s coastdown work utilized a flatbed trailer which weighed 10,480 pounds. Based on 

this, the agencies are defining a flatbed trailer weight of 10,500 pounds.

Lastly, a tanker trailer weight typically ranges between 9,010 and 10,500 pounds based 

on ICF findings. The tanker trailer used in the coastdown testing weighed 9,840 pounds. The 

agencies are defining the empty tanker trailer weight of 10,000 pounds.

3.5.3 Payload

The amount of payload by weight that a tractor can carry depends on the class (or 

GVWR) of the vehicle. For example, a typical Class 7 tractor can carry fewer tons of payload 

than a Class 8 tractor. Payload impacts both the overall test weight of the truck and is used to 

assess the “per ton-mile” fuel consumption and GHG emissions. The “tons” represent the 

payload measured in tons. 

M.J. Bradley analyzed the Truck Inventory and Use Survey and found that approximately

9 percent of combination tractor miles travelled empty, 61 percent are “cubed-out” (the trailer is 

full before the weight limit is reached), and 30 percent are “weighed out” (operating weight 

equal 80,000 pounds which is the gross vehicle weight limit on the Federal Interstate Highway

System or greater than 80,000 pounds for vehicles traveling on roads outside of the interstate 

system).25 The Federal Highway Administration developed Truck Payload Equivalent Factors to 

inform the development of highway system strategies using Vehicle Inventory and Use Survey

(VIUS) and Vehicle Travel Information System (VTRIS) data. Their results, as shown in Table 

3-27, found that the average payload of a Class 8 truck ranged from 29,628 to 40,243 pounds, 

depending on the average distance travelled per day.

26

 The same results found that Class 7 

trucks carried between 18,674 and 34,210 pounds of payload also depending on average distance 

travelled per day.

Table 3-27 National Average Payload (lbs.) per Distance Travelled and Gross Vehicle Weight Group 

(VIUS)27

CLASS 3 CLASS 4 CLASS 5 CLASS 6 CLASS 7 CLASS 8

< 50 miles 3,706 4,550 8,023 10,310 18,674 29,628

51 to100 miles 3,585 4,913 6,436 10,628 23,270 36,247

101 to 200 miles 4,189 6,628 8,491 12,747 30,180 39,743

201 to 500 miles 4,273 7,029 6,360 10,301 25,379 40,243

> 500 mile 3,216 8,052 6,545 12,031 34,210 40,089

Average 3,794 6,234 7,171 11,203 26,343 37,190

The agencies are prescribing a fixed payload of 25,000 pounds for Class 7 tractors and 

38,000 pounds for Class 8 tractors for their respective test procedures. These payload values 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-67

represent a heavily loaded trailer, but not maximum GVWR, since as described above the 

majority of tractors "cube-out" rather than "weigh-out.” 

NHTSA and EPA are also proposing to continue using the payload requirements for each 

regulatory subcategory in the vocational vehicle category that were finalized in the HD Phase 1 

rule. The payloads were developed from Federal Highway statistics based on the averaging the 

payloads for the weight classes of represented within each vehicle category.28 The payload 

requirement is 5,700 pounds for the Light Heavy trucks based on the average payload of Class 3, 

4, and 5 trucks from Table 3-27. The payload for Medium Heavy trucks is 11,200 pounds per 

the average payload of Class 6 trucks as shown in Table 3-27. Lastly the agencies are defining 

38,000 pounds payload for the Heavy Heavy trucks based on the average Class 8 payload in

Table 3-27. 

3.5.4 Total Weight

In summary, the total weights of the combination tractors are shown in Table 3-28.

Table 3-28 Combination Tractor Total Weight

MODEL 

TYPE

CLASS 8 CLASS 8 CLASS 8 CLASS 8 CLASS 8 CLASS 8 CLASS 7 CLASS 7 CLASS 7

Regulatory 

Subcategory

Sleeper 

Cab High 

Roof

Sleeper 

Cab Mid 

Roof

Sleeper 

Cab Low 

Roof

Day Cab 

High 

Roof

Day Cab 

Mid Roof

Day Cab 

Low Roof

Day Cab 

High 

Roof

Day Cab 

Mid Roof

Day Cab 

Low 

Roof

Tractor Tare 

Weight (lbs)

19,000 18,750 18,500 17,500 17,100 17,000 11,500 11,100 11,000

Trailer 

Weight (lbs)

13,500 10,000 10,500 13,500 10,000 10,500 13,500 10,000 10,500

Payload 

(lbs)

38,000 38,000 38,000 38,000 38,000 38,000 25,000 25,000 25,000

Total 

Weight (lbs)

70,500 66,750 67,000 69,000 65,100 65,500 50,000 46,100 46,500

The total weights of the vocational vehicles are as shown in Table 3-29.

Table 3-29 Vocational Vehicle Total Weights

REGULATORY 

SUBCATEGORY

LIGHT 

HEAVY

MEDIUM 

HEAVY

HEAVY

HEAVY

Truck Tare 

Weight (lbs)

10,300 13,950 27,000

Payload (lbs) 5,700 11,200 15,000

Total Weight (lbs) 16,000 25,150 42,000

3.6 Powertrain Test Procedures

In the HD Phase 1 rule the agencies introduced a powertrain test procedure to allow 

manufacturers to generate credits for selling advanced powertrains that reduced CO2 emissions 

and fuel consumption. In Phase 2 we are proposing to bring the powertrain test procedure into 

the main program and project that 15 to 30 percent of the vocational vehicles (including both 

hybrid and non-hybrid applications) would certify using this method. To accommodate this 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-68

change we are proposing a number of improvements to the test procedure in 40 CFR 1037.550 

and reducing the test burden by only requiring testing of the powertrain that is to be certified. 

The agencies are also proposing modifications to 40 CFR 1037.550 to separate out the hybrid 

specific testing protocols.

3.6.1 Reason Behind Use Of Powertrain Test Method for Conventional and Hybrid 

Powertrain Certification

The agencies are proposing a powertrain test option to afford a robust mechanism to 

quantify the benefits of CO2 reducing technologies that are a part of the powertrain (conventional 

or hybrid), that are not captured in the GEM simulation. Among these technologies are transient 

fuel control, engine and transmission control integration, and hybrid systems. The largest 

proposed change from the Phase 1 powertrain procedure is that only the advanced powertrain 

will need to be tested as opposed to the requirement in Phase 1 where the result was an 

improvement factor calculated from the powertrain results of both the advanced powertrain and 

the conventional powertrain. This proposed change is possible because the proposed GEM 

simulation tool uses the engine fuel map and torque curve from the actual engine in the vehicle 

that is to be certified for all vehicles that do not use the powertrain method to certify the vehicle.

3.6.2 Use of Generic Vehicles to Apply Measurements Broadly Across All Vehicles

That the Powertrain Will Be Installed In

To limit the amount of testing, under the proposal, powertrains will be divided into 

families and will be tested in a limited number of simulated vehicles that will cover the range of 

vehicles in which the powertrain will be used.

A matrix of 8 to 9 tests would be needed per vehicle cycle, to enable the use of the 

powertrain results broadly across all the vehicles in which the powertrain will be installed. The 

individual tests differ by the vehicle that is being simulated during the test. Table 3-30 and Table 

3-31 define the unique vehicles being proposed that would cover the range of coefficient of drag, 

coefficient of rolling resistance, vehicle mass and axle ratio of the vehicles that the powertrain 

will be installed in.

To allow for a generic tire size definition that will cover the tires and axles installed on 

the certified vehicles, the agencies are proposing that each tire radius will be set so that when the 

vehicle is cruising at 65 mph the engine speed will equal the corresponding minimum NTE 

exclusion speed as defined in 40 CFR part 86.1370(b)(1), intermediate test speed (A, B, or C), or 

maximum test speed defined in 40 CFR part 1065. To calculate the tire radius, use the equation 

in 40 CFR 1037.550.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-69

Table 3-30 Proposed Generic Vehicle Definitions for Class 2b-7 Vehicles

TEST 1 TEST 2 TEST 3 TEST 4 TEST 5 TEST 6 TEST 7 TEST 8

Mass (kg) 7,257 11,408 7,257 11,408 7,257 11,408 7,257 11,408

CdA 5.4 5.4 5.4 5.4 5.4 5.4 5.4 5.4

Tire Crr

(kg/ton)

6.7 6.9 6.7 6.9 6.7 6.9 6.7 6.9

Tire Radius 

(m)

0.426 0.426 0.426 0.426 0.426 0.426 0.426 0.426

Rotating 

Inertia (kg)

454 454 454 454 454 454 454 454

Axle Gear 

Efficiency (%)

95.5 95.5 95.5 95.5 95.5 95.5 95.5 95.5

Accessory 

Power (W) 

1300 1300 1300 1300 1300 1300 1300 1300

Axle ratio at 

engine speed

A A B B C C Maximum 

engine 

speed

Maximum 

engine 

speed

Table 3-31 Proposed Generic Vehicle Definitions for Tractors and Class 8 Vocational Vehicles—General 

Purpose

TEST 1 TEST 2 TEST 3 TEST 4 TEST 5 TEST 6 TEST 7 TEST 8 TEST 9

Mass (kg) 31,978 22,679 19,051 31,978 22,679 19,051 31,978 22,679 19,051

CdA 5.4 4.7 4.0 5.4 4.7 4.0 5.4 4.7 4.0

Tire Crr

(kg/ton)

6.9 6.9 6.9 6.9 6.9 6.9 6.9 6.9 6.9

Tire 

Radius 

(m)

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

Rotating 

Inertia

(kg)

1,134 907 680 1,134 907 680 1,134 907 680

Axle Gear 

Efficiency 

(%)

95.5 95.5 95.5 95.5 95.5 95.5 95.5 95.5 95.5

Accessory 

Power

(W) 

1300 1300 1300 1300 1300 1300 1300 1300 1300

Axle ratio 

at engine 

speed

Minimum 

NTE 

exclusion 

speed

Minimum 

NTE 

exclusion 

speed

Minimum 

NTE 

exclusion 

speed

B B B Maximum 

engine 

speed

Maximum 

engine 

speed

Maximum 

engine

speed
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-70

Table 3-32 Proposed Generic Vehicle Definitions for Class 8 Combination— Heavy-Haul Vehicle

TEST 1 TEST 2 TEST 3 TEST 4 TEST 5 TEST 6 TEST 7 TEST 8 TEST 9

Mass (kg) 40,895 31,978 22,679 40,895 31,978 22,679 40,895 31,978 22,679

CdA 6.1 5.4 4.7 6.1 5.4 4.7 6.1 5.4 4.7

Tire Crr

(kg/ton)

6.9 6.9 6.9 6.9 6.9 6.9 6.9 6.9 6.9

Tire 

Radius 

(m)

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

Rotating 

Inertia

(kg)

1,134 907 680 1,134 907 680 1,134 907 680

Axle Gear 

Efficiency 

(%)

95.5 95.5 95.5 95.5 95.5 95.5 95.5 95.5 95.5

Accessory 

Power

(W) 

1300 1300 1300 1300 1300 1300 1300 1300 1300

Axle ratio 

at engine 

speed

Minimum 

NTE 

exclusion 

speed

Minimum 

NTE 

exclusion 

speed

Minimum 

NTE 

exclusion 

speed

B B B Maximum 

engine 

speed

Maximum 

engine 

speed

Maximum 

engine 

speed

The main outputs of this matrix of tests is grams of CO2, the average transmission output 

shaft speed divided by the average vehicle speed and positive work measured at the output shaft 

of the powertrain. This matrix of test results will then be used to calculate the vehicle’s CO2

emissions in GEM taking the work per ton-mile from the GEM simulation and multiplying it by

the interpolated work specific CO2 mass emissions from the powertrain test.

3.6.3 Measurement Method and Results

The agencies are proposing to expand upon the test procedures defined 40 CFR 1037.550 

for HD Phase 1. The Phase 2 proposed expansion will migrate the current Phase 1 test procedure 

to a new 40 CFR 1037.555 and will modify the current test procedure in 40 CFR 1037.550, 

allowing its use for Phase 2 only. The Phase 2 modifications to 40 CFR 1037.550 include the 

addition of the rotating inertia of the driveline and tires, the axle efficiency and the vehicle’s 

accessory loads. This revised procedure also requires that each of the powertrain components be 

cooled so that the temperature of each of the components is kept in the normal operation range. 

In addition to changing the vehicle model, we are proposing changes to the drive model 

so that it can compensate when the powertrain gets ahead or falls behind in the duty cycle. Use 

of this compensation algorithm will ensure that every powertrain drives the complete distance of 

the cycle, regardless of whether or not it can maintain the target speed of the cycle at a given 

moment in time. 

Although detailed equations for the vehicle and driver models can be found in the 

proposed 40 CFR 1037.550, the agencies are recommending that manufactures use the 

MATLAB and Simulink models provided by the agencies. These models can be found at 

http://www.epa.gov/otaq/climate/gem.htm.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-71

Conventional Powertrain Test Results

The agencies have performed internal test programs, contracted with outside labs, as well 

as collaborated with manufactures to test out the improvements to the powertrain test procedure. 

The data presented in Figure 3-10 is from a conventional powertrain that consisted of a Cummins 

ISX engine and Eaton 10 speed automated manual transmission that was tested in one of these 

test programs. This data summarizes the results from three different types of tests. The first set 

of data, labeled “Engine Only”, was collected from engine tests where the speed and torque 

setpoints were determined by GEM. The simulations were done with 9 different vehicle 

configurations over the three duty cycles that are being proposed as certification duty cycles (55

mph with grade, 65 mph with grade and ARB transient cycle). The “GEM Model” data contains

the CO2 emissions as determined by GEM using the engine’s fuel map and the transmission’s 

gear ratios using with the default shift strategy. The x-axis defines the Powertrain test results. 

The data shows that across all three test cycles the powertrain test procedure produces 2.5

percent less CO2 emission than the GEM simulation predicted. One must, however, take into 

account the fact that the GEM simulation was done using the engine steady-state fuel map; thus,

the GEM results don’t fully take into account the effect of transient fueling on CO2 emissions. 

This is evident when looking at the data collected when operating over the transient test cycles

(highest CO2 g/ton-mile results). Here you see that the engine consumed greater than 3 percent

more fuel than GEM predicted. When taking the transient test results into account, the 

powertrain performed 5 to 8 percent better than GEM predicted.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-72

Figure 3-10 Engine only and GEM CO2 results vs. powertrain.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-73

3.6.4 Powertrain Family Definition

To complement the agencies powertrain procedures we are proposing criteria for defining 

a powertrain family. The specifics of these criteria can be found in 40 CFR 1037.231 but 

nominally a powertrain family is made up of one engine family and one transmission family.

3.6.4.1 Criteria for Powertrain Families

The proposed regulations in 40 CFR 1037.231 outline the criteria for grouping 

transmission models into powertrain families sharing similar emission characteristics. A few of 

these defining criteria include the transmission’s architecture (manual, automatic, automated 

manual, dual-clutch and hybrid), number of gears in the front box, number of meshes in the back 

box and dry sump versus regular sump. In addition to the criteria for the transmission, all the 

engines in the powertrain family have to be from the same engine family.

3.6.4.2 Emissions Test Powertrain

We are proposing that manufacturers select at least one powertrain per powertrain family

for emission testing. The methodology for selecting the test powertrain(s) should be consistent 

with 40 CFR 1037.231. The test powertrain(s) should consist of the engine and transmission 

combination that results in the highest CO2 emissions.

3.6.5 Vehicle Certification with Powertrain Results in GEM

For manufactures that choose to use the powertrain method when certifying a vehicle, the 

powertrain results from the test will be input into GEM instead of the engine’s fuel map, torque 

curve, motoring curve and the transmissions gear ratios. GEM will use the default powertrain 

inputs, as described in Table 3-33, and the inputs of the to-be certified vehicle to calculate the 

cycle work (W) of the powertrain and the ratio of rotational speed over the vehicle speed (N/V) as 

defined by the tire radius and rear-axle ratio.

Table 3-33 GEM Default Parameters for Vehicle Certification Using Powertrain Testing.

REGULATORY CLASS ENGINE TRANSMISSION GEAR RATIOS

Class 8 

Combination

Heavy-Haul 2017 MY 15L 

Engine with 600 

HP

13 speed 

Automated Manual 

Transmission

12.29, 8.51, 

6.05, 4.38, 3.20, 

2.29, 1.95, 1.62, 

1.38, 1.17, 1.00, 

0.86, 0.73

Sleeper Cab - High Roof 2017 MY 15L 

Engine with 455 

HP

10 speed 

Automated Manual 

Transmission

12.8, 9.25, 6.76, 

4.9, 3.58, 2.61, 

1.89, 1.38, 1, 

0.73

Sleeper Cab - Mid Roof

Sleeper Cab - Low Roof

Day Cab - High Roof

Day Cab - Mid Roof

Day Cab - Low Roof

Class 7 Day Cab - High Roof 2017 MY 13L 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-74

Combination Day Cab - Mid Roof Engine with 350 

HP

Day Cab - Low Roof

HHD 

Vocational

Regional Duty Cycle 2017 MY 15L 

Engine with 455 

HP

Multi-Purpose Duty Cycle 2017 MY 11L 

Engine with 345 

HP

5 speed HHD 

Automatic 

Transmission

4.6957, 2.213,

1.5291, 1,

0.7643 Urban Duty Cycle 

MHD 

Vocational

Regional Duty Cycle 2017 MY 7L 

Engine with 270 

HP

5 speed MLHD 

Automatic 

Transmission

3.102, 1.8107,

1.4063, 1,

Multi 0.7117 -Purpose Duty Cycle 

Urban Duty Cycle 

LHD 

Vocational

Regional Duty Cycle 2017 MY 7L 

Engine with 200 

Multi HP -Purpose Duty Cycle 

Urban Duty Cycle 

In GEM the cycle work from the powertrain testing will be corrected for the electrical 

and mechanical accessory power according to the following equation. The accessory power is 

defined for each vehicle category in Chapter 4 of this RIA.

  =  −  ∙  ∙

.   ()

()

GEM will use the calculated cycle work and N/V of the powertrain for the to-be certified 

vehicle to interpolate the powertrain input table. For vehicle configurations that have cycle work 

or N/V outside of the powertrain input table, we are proposing that the closest end points of the 

table be used instead of extrapolating. GEM will then use the following equation to calculate the 

CO2 g/ton-mile result per cycle before any technology inputs are applied. Finally the technology

inputs are applied, all the cycles are weighted and the gallons of fuel are then calculated from the 

mass of CO2.

 =  



h





∙  ∙

1

 ∙  ∙





3.7 Hybrid Powertrain Test Procedures

As discussed in Section V of the preamble, the agencies see an opportunity to help drive 

the technology’s advancement by predicating the vocational vehicle standards on a small 

adoption rate of hybrid powertrains in this rulemaking. However, since the projected 

effectiveness of this technology over the proposed Urban vocational duty cycle is 25 percent, the 

agencies believe it is no longer appropriate to provide a 1.5 multiplier for credits generated by 

vehicles applying this technology. EPA and NHTSA are proposing two methods to demonstrate 

benefits of a hybrid powertrain – chassis and engine testing.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-75

3.7.1 Measurement Method and Results

The agencies are proposing that hybrid powertrains be tested just like conventional 

powertrains, with the dynamometer connected at either the input shaft of the rear axle or the 

input shaft to the wheels, using the powertrain method described in Section 3.6 with some 

additional requirements for the rechargeable energy storage systems (RESS) net energy change 

(NEC) over the test.

As in the Phase 1 rule, the agencies are proposing that hybrids will be tested under charge 

sustain operation so that all the energy to drive the cycle comes from the on board hybrid 

powertrain. The NEC of the RESS must meet the requirements of SAE J2711 for each test.

3.7.2 Engine Hybrid Method

To address hybrid powertrain system performance for hybrids that recover energy

between the engine and transmission, the agencies are proposing to retain the engine hybrid 

procedures defined in 40 CFR 1036.525. The control volume for these hybrids is drawn so as to 

include the battery, battery support and control systems, power electronics, the engine, motor 

generator and hybrid control module. The performance of this system is an engine based 

evaluation in which emission rates are determined on a brake-specific work basis. As such, the 

duty cycles being proposed to assess this system performance are engine speed and torque 

command cycles that are similar but not identical to the cycles used for criteria pollutant

standards. In addition to the cycles being slightly different between the test for GHG emissions 

and the test for criteria emissions, the system boundary of the engine for the criteria emission test 

will remain unchanged and will not include the hybrid components. It is expected that, parallel

engine hybrids would be the most likely choice for engine-based hybrid certification. Details 

related to engine hybrid test procedures may be found in 40 CFR 1036.525.

 

Figure 3-11 Engine Hybrid Test Configuration
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-76

3.7.3 Removal of the Chassis Test Option for Hybrids

In the Phase 1 rule the agencies finalized a powertrain and chassis test option for hybrid 

testing. The agencies are proposing to remove the chassis test option for the Phase 2 program 

because it appears to be incompatible with the proposed changes regarding use of results from 

the hybrid test procedure. In the proposed procedure, the output of the hybrid test is brake 

specific CO2 emission where the positive work is measured at the output shaft of the hybrid 

powertrain. Since work cannot be measured at this location on a chassis dynamometer without 

modifying the vehicle, the agencies are proposing the removal of the chassis testing option. 

Another reason for the removal of the chassis test option is that there are a number of additional 

sources of variability when testing a vehicle on a chassis dynamometer. These include electrical 

and mechanical accessory load, tire temperature and driver variability to name a few.

3.7.4 Electrified PTO Test Method

A power take off (PTO) is a system on a vehicle that allows energy to be drawn from the 

vehicle’s drive system and used to power an attachment or a separate machine. Typically in a 

heavy-duty truck, a shaft runs from the transmission of the truck and operates a hydraulic pump. 

The operator of the truck can select to engage the PTO shaft in order for it to do work, or 

disengage the PTO shaft when the PTO is not required to do work. The pressure and flow from 

this hydraulic fluid can be used to do work in implements attached to the truck. Common 

examples of this are utility trucks that have a lift boom on them, refuse trucks that pick up and 

compact trash, and cement trucks that have a rotating barrel. In each case the auxiliary

implement is typically powered by a PTO that uses energy from the truck’s primary drive engine.

In most PTO equipped trucks, it is necessary to run the primary drive engine at all times 

when the PTO might be needed. This is an unoptimized configuration. Typical PTO systems 

require no more than 19 kW at any time, which is far below the optimal operation range of the 

primary drive engine of most trucks. Furthermore, in intermittent operations, the primary drive 

engine is kept running at all times in order to ensure that the PTO can operate instantaneously. 

This results in excess GHG emissions and fuel consumption due to idle time. Additionally, 

idling a truck engine for prolonged periods of time while operating auxiliary equipment like a 

PTO could cause the engine to cycle into a higher idle speed, wasting even more fuel. 

Hybridization and changing the operation of a conventional PTO equipped truck are two 

viable means to lower the GHG emissions and fuel consumption in the real world. The proposed 

test procedures will allow for manufactures to quantify the reduction of CO2 emissions and fuel 

consumption from electrified PTO systems.

In Phase 1, hybrid PTO testing was performed either via chassis or powertrain testing of 

both the conventional and hybrid systems over the PTO duty cycles described in Appendix II of 

40 CFR 1037, in addition to the vehicle duty cycles. An improvement factor was then generated

as described in 40 CFR 1037.615 and applied to the g/ton-mile CO2 emission rate resulting from 

the GEM output for the advanced vehicle as described in 40 CFR 1037.520.

EPA and NHTSA are proposing to continue the Phase 1 testing methodology outlined in 

40 CFR 1037.525 where A to B testing is used to generate an improvement factor either via 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-77

powertrain or chassis testing. The one change that the agencies are proposing for Phase 2 is how 

the results are used to calculate the vehicle’s emission result. For Phase 2, the agencies are 

proposing that the reduction in emissions from the electrified PTO system versus the 

conventional PTO system be subtracted from the composite driving emissions result. Specifics 

on the applicability of electrified PTOs is discussed further in Chapter V. C of the preamble.

3.8 Rear Axle Efficiency Test

In Phase 2, the agencies developed test procedures to measure axle efficiency. See 40 

CFR 1037.515. This procedure ultimately provides for the determination of torque loss versus 

input speed and input torque for use in the GEM simulation tool. The procedure provides 

limitations on axle break in procedures and prescribes dynamometer set ups for axles with and 

without lockable differentials as well as drive-through axles. This procedure puts limitations on 

the test cell ambient temperature, sump oil temperature, and requires the use of representative 

commercially available axle lubricating oil. The mapping process requires that you map the axle 

by testing with an input torque in the range of 0 to 4000 Nm in 1000 Nm steps at wheel speeds 

that range from 50 rpm to the maximum wheel speed in 50 rpm steps. The procedure sweeps 

though the torque points at a given wheel speed from the minimum to the maximum torque point 

and back, with the process repeated twice for a given wheel speed. The four values generated at 

each speed and torque point are then averaged resulting in one map point per wheel and output 

torque value.

3.9 HD Pickup Truck and Van Chassis Test Procedure

The agencies are proposing that HD pickup trucks and vans continue to demonstrate 

compliance using the 40 CFR part 1066 chassis test procedures. For each test vehicle from a 

family required to comply with the GHG and fuel consumption requirements, the manufacturer 

would supply representative road load forces for the vehicle at speeds between 15 km/hr (9.3 

mph) and 115 km/hr (71.5 mph). The road load force would represent vehicle operation on a 

smooth level road, during calm winds, with no precipitation, at an ambient temperature of 20 °C 

(68 °F), and atmospheric pressure of 98.21 kPa. Road load force for speeds below 9.3 mph may

be extrapolated.

The dynamometer's power absorption would be set for each vehicle's emission test 

sequence such that the force imposed during dynamometer operation matches actual road load 

force at all speeds. Required test dynamometer inertia weight class selections are determined by

the test vehicle test weight basis using adjusted loaded vehicle weight from which the

corresponding equivalent test weight is determined.

3.9.1 LHD FTP and HWFE Testing

The FTP dynamometer schedule consists of two tests, a “cold” start UDDS test after a 

minimum 12-hour and a maximum 36-hour soak according to the provisions of 40 CFR 

1066.801, 1066.815, and 1066.816, and a “hot” start test following the “cold” start after a 10 

minute soak. Engine startup (with all accessories turned off), operation over the UDDS, and 

engine shutdown constitutes a complete cold start test. Engine startup and operation over the 

first 505 seconds of the driving schedule complete the hot start test. The driving schedule for 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-78

EPA’s Urban Dynamometer Driving Schedule is contained in Appendix I of 40 CFR part 86. 

The driving schedule is defined by a smooth trace drawn through the specified speed versus time 

relationship. The schedule consists of a distinct non-repetitive series of idle, acceleration, cruise, 

and deceleration modes of various time sequences and rates.

The Highway Fuel Economy Dynamometer Procedure (HFET) consists of 

preconditioning highway driving sequence and a measured highway driving sequence. The 

HFET is designated to simulate non-metropolitan driving with an average speed of 48.6 mph and 

a maximum speed of 60 mph. The cycle is 10.2 miles long with 0.2 stops per mile and consists 

of warmed-up vehicle operation on a chassis dynamometer through a specified driving cycle. 

The Highway Fuel Economy Driving Schedule is set forth in Appendix I of 40 CFR Part 600, 

while the test is carried out according to 40 CFR 1066.840. The driving schedule is defined by a 

smooth trace drawn through the specified speed versus time relationships.

Practice runs over the prescribed driving schedules may be performed, provided an 

emission sample is not taken, for the purpose of finding the appropriate throttle action to 

maintain the proper speed-time relationship, or to permit sampling system adjustment. 

Both smoothing of speed variations and excessive accelerator pedal perturbations are to be 

avoided. The driver should attempt to follow the target schedule as closely as possible. The 

speed tolerance at any given time on the dynamometer driving schedules specified in Appendix I

of parts 86 and 600 is defined by upper and lower limits in 40 CFR 1066.425. The upper limit is 

2 mph higher than the highest point on trace within 1 second of the given time. The lower limit 

is 2 mph lower than the lowest point on the trace within 1 second of the given time. Speed 

variations greater than the tolerances (such as may occur during gear changes) are acceptable 

provided they occur for less than 2 seconds on any occasion. Speeds lower than those prescribed 

are acceptable provided the vehicle is operated at maximum available power during such 

occurrences.

3.9.2 LHD FTP and HWFE Hybrid Testing

Since LHD chassis certified vehicles share test schedules and test equipment with much 

of Light-Duty Vehicle testing, EPA believes it is appropriate to continue to use the HD Phase 1 

test procedure which references SAE J1711 “Recommended Practice for Measuring the Exhaust 

Emissions and Fuel Economy of Hybrid-Electric Vehicles, Including Plug-in Hybrid Vehicles” 

instead of SAE J2711 “Recommended Practice for Measuring Fuel Economy and Emissions of 

Hybrid-Electric and Conventional Heavy-Duty Vehicles”.

3.9.2.1 Charge Depleting Operation – FTP or “City” Test and HFET or 

“Highway” Test

EPA would like comment on incorporating by reference SAE J1711 Chapters 3 and 4, as 

published June 2010, testing procedures for Light-Heavy-Duty chassis certified vehicles with the 

following exceptions and clarifications:

Test cycles will continue, until the end of the phase of the test cycle, in which charge 

sustain operation is confirmed. Charge sustain operation is confirmed when one or more phases 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-79

or cycles satisfy the Net Energy Change requirements below. Optionally, a manufacturer may

terminate charge deplete testing before charge sustain operation is confirmed provided that the 

Rechargeable Energy Storage System (RESS) has a higher State of Charge (SOC) at charge 

deplete testing termination than in charge sustain operation. In the case of Plug-In Hybrid 

Electric Vehicles (PHEV) with an all-electric range, engine start time will be recorded but the 

test does not necessarily terminate with engine start. PHEVs with all electric operation follow 

the same test termination criteria as blended mode PHEVs. Testing can only be terminated at the 

end of a test cycle. The regulation allows EPA to approve alternate end of test criteria as 

described in 40 CFR 1066.501.

For the purposes of charge depleting CO2 and fuel efficiency testing, manufacturers may

elect to report one measurement per phase (one bag per UDDS). Exhaust emissions need not be 

reported or measured in the phases of the test the engine does not operate.

End of test recharging procedure is intended to return the RESS to a full charge 

equivalent to pretest conditions. The recharge AC watt hours must be recorded throughout the 

charge time and soak time. Vehicle soak conditions must not be violated. The AC watt hours 

must include the charger efficiency. The measured AC watt hours are intended to reflect all 

applicable electricity consumption including charger losses, battery and vehicle conditioning 

during the recharge and soak, and the electricity consumption during the duty cycles.

Net Energy Change Tolerance (NEC), is to be applied to the RESS to confirm charge 

sustaining operation. The agencies are proposing to continue to use the 1 percent of fuel energy

NEC state of charge criteria as expressed in SAE J1711 and described in 40 CFR 1066.501. The 

Administrator may approve alternate NEC tolerances and state of charge correction factors.

3.9.2.2 Hybrid Charge Sustaining Operation – FTP or “City” Test and HFET 

or “Highway” Test

The agencies are proposing to continue incorporating by reference SAE J1711 Chapters 3 

and 4 for definitions and test procedures, respectively, where appropriate, with the following 

exceptions and clarifications. 

The agencies are adopting the 1 percent of fuel energy NEC state of charge criteria as 

expressed in SAE J1711 and described in 40 CFR 1066.501. The Administrator may approve 

alternate NEC tolerances and state of charge correction factors.

Preconditioning special procedures are optional for traditional “warm” test cycles that are 

now required to test starting at full RESS charge due to charge depleting range testing. If the 

vehicle is equipped with a charge sustain switch, the preconditioning cycle may be conducted per 

40 CFR 600.111 provided that the RESS is not charged. Exhaust emissions are not taken in 

preconditioning drives. Alternate vehicle warm up strategies may be approved by the 

Administrator. 

State of Charge tolerance correction factors may be approved by the Administrator as 

described in 40 CFR 1066.501. RESS state of charge tolerances beyond the 1 percent of fuel 

energy may be approved by the Administrator.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-80

The agencies are seeking comment on modifying the minimum and maximum allowable 

test vehicle accumulated mileage for both EVs and PHEVs. Due to the nature of PHEV and EV 

operation, testing may require many more vehicle miles than conventional vehicles. 

Furthermore, EVs and PHEVs either do not have engines or may use the engine for only a 

fraction of the miles driven.

Electric Vehicles and PHEVs are to be recharged using the supplied manufacturer 

method provided that the methods are available to consumers. This method could include the 

electricity service requirements such as service amperage, voltage, and phase. Manufacturers 

may employ the use of voltage regulators in order to reduce test to test variability with prior 

Administrator approval.

3.10 Alternative Certification Approach

3.10.1 Purpose and Scope

Under the Phase 1 rule, vocational vehicles and tractors are certified by using GEM with 

the default engine fuel map pre-defined by the agency, while the engine is certified by either 

using the SET cycle for tractor engines or the FTP cycle for vocational engines, which are totally

different from vehicle drive cycles. 

In this section, a new concept as an alternative to the engine fuel mapping test, proposed 

in Phase 2 is explored. This approach would allow use of the same drive cycles for both the 

vehicle and engine compliance process, without the need for engine manufacturer providing the 

steady state engine fuel map for vehicle certification. Therefore, this approach has the potential 

to totally integrate vehicle and engine certification, while more accurately quantifying the 

transient engine operation than is possible using a traditional steady state engine fuel map.

The potential approach discussed here would be an alternative to certifying vocational 

vehicles, tractors, and their engines using a steady-state fuel map. The agencies solicit comment 

on this alternative, and commenters should include their thoughts on whether this concept can be 

adequately fleshed out in the time remaining in this rulemaking. 

3.10.1.1 Phase 1 Certification Approach

In order to help to understand the vocational vehicle and tractor vehicle certification 

process under the Phase 1 rule, Figure 3-12 summarizes the GEM-based certification process 

flow.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-81

Figure 3-12 Phase 1 Rule for Certification Using GEM
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-82

In this approach, vehicle manufacturers can only use up to five input parameters – aero 

dynamic coefficient, rolling resistance, weight reduction, speed limiter, and idle reduction, to 

conduct vehicle certifications (although improvements not recognized by GEM can be certified 

as off-cycle credits under the Phase 1 rules). All other vehicle and engine parameters are the 

default parameters specified by the agencies. 

Figure 3-12 shows that use of the agencies’ default engine fuel maps under the Phase 1 

rules. This default engine fuel map approach would not be able to recognize the benefits of 

advanced engine technologies packages. The three drive cycles shown in Figure 3-12used in 

Phase 1 are 55 mph and 65 mph cruise speed cycles, and ARB transient cycles. In the two cruise 

speed cycles, there is no road grade, meaning that the vehicle operates with one single operating 

point inside engine fuel maps, which does not represent the real-life driving condition on the 

road. On the other hand, engine certifications use completely different cycles. For example, 

tractor engines use the SET cycle, while vocational engines use the FTP cycle. Those engine 

cycles have very little connection with how the engine operates in a vehicle over Phase 1 vehicle 

drive cycles. This means whatever the optimized engine calibration developed for the FTP and 

SET cycles cell may not be able to be realized in the real-life driving condition on the road. 

Furthermore, there is no direct linkage between GHG emissions and criteria emissions, since 

vehicle and engine certification cycles are different. 

3.10.1.2 Primary Certification Approach in Phase 2

In order to overcome the deficiencies mentioned in the above section, the agencies are 

proposing significant improvements and enhancements as part of the Phase 2 proposal. Many of 

the Phase 1 predefined parameters used in GEM now become the vehicle-specific user-entered 

inputs in Phase 2. Most significantly, vehicle manufacturers can use engine fuel maps 

representing the actual engine in the vehicle for certification, which means that it can recognize 

the benefits due to advanced technologies developed for the engines. Chapter 4 of this draft RIA 

details the enhancements of GEM and the extensive validations of these proposed changes. 

Another significant proposed enhancement is the addition of the road grade into 55 mph and 65 

mph cruise speed cycles, which represents more realistic driving conditions. In addition, the 

agencies propose reweighting on the SET cycles with more emphasis on A and B speed modes. 

The more detailed description on addition of the SET and road grade weighting can be seen in 

Section II.D(1) and Section III.C of the preamble, respectively. 

Even with these improvements, however, certain issues would not be directly addressed. 

First of all, there would still be no direct linkage between GHG emissions and criteria pollutant 

emissions, since vehicle and engine certification cycles would still be different. Second, the 

engine fuel maps used in GEM from individual vehicle manufacturers are still obtained under 

steady state conditions. The transient behaviors due to smoke control and thermal management 

control, for example, would not be able to be modeled using steady state engine fuel maps. 

Third, the agencies’ primary certification approach introduces a new concern from independent 

engine manufacturers, namely concern regarding proprietary technology information that can be 

found from engine fuel maps.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-83

3.10.2 Description of Alternative Certification Concept

In view of these concerns mentioned above two sections for both Phase 1 and Phase 2, 

the agencies would like to specifically ask for comments on this alternative approach to Phase 2 

certification to address these concerns. This section will introduce the concept and principal of 

this alternative approach.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-84

3.10.2.1 Vehicle Certification

Figure 3-13 Alternative Phase 2 Certification Option II

Figure 3-14 Phase 2 Certification Process with Option II
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-85

Displayed in Figure 3-13 and Figure 3-14 is the potential alternative vehicle certification 

approach. The entire process can be further simplified and described as follows. 

1. Define nine vehicles that will cover the range of vehicles that the engine will be 

certified in. In one dimension the vehicles will cover the range of engine cycle 

work (W) by varying the vehicles mass, CdA and Crr. The other dimension would 

cover the range of average engine speed over average vehicle speed (N/V), by

varying the vehicles tire size and rear axle ratio or transmission (see the left part 

of Figure 3-13). 

2. Run GEM with the nine vehicle configurations and the three certification cycles 

to generate 27 engine cycles (three certification cycles multiplied by nine 

vehicles). Each cycle will define engine speed and torque as a function of time. 

These cycles will be generated at 10hz to capture engine torque during shifting 

(see the right part of Figure 3-13).

3. Test the parent engine of the engine family using the 27 engines cycles to create a 

matrix of brake specific CO2 consumption in g/hp-hr as a function of Work in hphr and N/V in rpm/(mile/hr). 

4. For certification, run GEM for each vehicle that will be certified with an engine 

from the engine family. For each simulation the actual vehicle parameters shall 

be used. The output of the simulation will be work in hp-hr and N/V in 

rpm/(mile/hr) for each drive cycle (See the left part of Figure 3-14 ).

5. Use work and N/V obtained from Step 4 to interpolate CO2 in g/hp-hr for the 

engine in the specific vehicle being certified (see the middle part of Figure 3-14.

6. Multiply the interpolated CO2 in g/hp-hr by work in (hp-hr) from the simulation 

of the certified vehicle (Step 4) and divide by the ton-miles of each vehicle 

category and drive cycle (see the right part of Figure 3-14). 

7. Supply this final CO2 in g/ton-mile for certification.

3.10.2.2 Engine Certification

One of the key features for the alternative vehicle certification approach is that 

supplementation of the engine tests requires running a number of tests at a certified engine dyno. 

These tests will result in a fuel consumption or CO2 map in g/kw-hr as function of cycle work 

and N/V over three certification cycles. Therefore, the engine certification point could be 

selected from one of the testing points through this fuel consumption or CO2 map, and therefore 

there is no need to run engine certification test alone, thus reducing engine test burden from 

manufacturers as far as GHG emission certification is concerned. How this single point is to be 

selected is something we need to work out in the near future.

3.10.3 Discussion of Alternative Certification Approach

The previous section only describes the principles of this alternative approach. This 

section will provide more detailed supporting information, addressing the following questions: 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-86

 Would the fuel consumption or CO2 maps be very well behaved, so that the 

interpolation or curve fitting can be carried out without losing accuracy?

 Would one map that combines all three certification cycles be adequate to represent 

the engine or would an individual map for each cycle be needed?

 What would be the most suitable independent axis of the fuel consumption maps to be 

used to minimize the impact of type of transmissions and their shifting strategies?

 What is the minimum number of engine tests and GEM simulations required to cover 

the range of vehicles certified for a given engine family?

In order to answer the above questions, a test matrix based on a Class 8 Kenworth T700 

tractor with a Cummins ISX engine was carefully designed as follows:

• 32 variations of the Kenworth T700

– Axel ratio: 2.64, 3.36,3.9 and 4.56

– Cd: 0.35, 0.45, 0.55, 0.65, and 0.75

– Crr: 0.005, 0.006, 0.007 and 0.008

• Cycles – with distance correction 

– 55 w/SwRI grade profile

– 65mph w/SwRI grade profile

– ARB Transient

• Transmissions – Eaton AMT

– 10 speed: F016E310C-LAS

– 13 speed: FO_16E313A_MHP

– 18 speed: FO-16E318B-MXP 

• Transmission shift strategy

– Eaton’s table shift

– EPA’s shift optimizer

In addition, a child rating engine is selected to show its impacts on the accuracy of this 

approach. This results in a total of 1,152 simulation runs.

3.10.4 GEM Simulations

Phase 2 GEM (as proposed) was used by EPA to perform a large scale of simulations 

based on the matrix proposed above. The results and plots are arranged as follows. Figure 3-15

to Figure 3-17 display the BSFC (g/hp-hr) surface plots over 55mph, 65mph, and ARB cycles 

with all points simulated. The next section discusses the evaluation of the results with the engine

child ratings.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-87

Figure 3-15 Contour plot of CO2 as function of cycle work and N/V for 55 mph cycle 

Figure 3-16 Contour plot of CO2 as function of cycle work and N/V for 65 mph cycle 

20

25

30

20 25 30 35 40 45

140

150

160

170

180

190

200

Work (hp-hr)

55mph

N/V (rpm/(km/hr))

bsfc (g/hp-hr)

bsfc

450hp rating

15

20

25

30

25 30 35 40 45 50

140

150

160

170

180

190

200

Work (hp-hr)

65mph

N/V (rpm/(km/hr))

bsfc (g/hp-hr)

bsfc

450hp rating
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-88

Figure 3-17 Contour plot of CO2 as function of cycle work and N/V for ABB cycle 

As can be seen from these three figures (Figure 3-15 to Figure 3-17), a surface is used to 

curve fit all points. Also shown in these figures is how well the actual points are fitted with the 

surface. It is seen from Figure 3-15 and Figure 3-17 that all test cycles are collapsed into one 

surface with different transmissions, different shifting strategies, and axles. The behavior that all 

simulation points are very well fitted into one surface plane suggests that impact of transmissions 

including shifting strategies and numbers of gears, axle ratio be minimal if the plots are designed 

in such a way that average engine speed (N) over average vehicle speed (V) defined as N/V is 

selected. 

Figure 3-18 shows the three-dimensional surface plot of BSFC as function of cycle work 

and N/V with all cycles combined into one plot. It can be seen that all points are not very well 

fitted into one surface plot, suggesting that certification be done in an individual cycle manner.

60

70

80

90

11.5 12 12.5

13 13.5

14 14.5

170

175

180

185

190

195

200

205

210

Work (hp-hr)

ARB

N/V (rpm/(km/hr))

bsfc (g/hp-hr)

bsfc

450hp rating
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-89

Figure 3-18 Surface plot of CO2 as function of cycle work and N/V for all three cycles

The simulations are then carried out in order to address child rating impact on this surface 

fit with each cycle. Figure 3-19 to Figure 3-21 show the behavior of all three individual cycles. 

It can be seen that all parent and child rating points are still collapsed into one surface plot, 

which is very similar to the results where only parent ratings are shown in Figure 3-16 to Figure 

3-18, suggesting that the same interpolation schemes or the same surface fitting could be applied 

to both parent and child ratings for those points that are located between the testing points.

20 40

60 80 10

20

30

40

50 140

160

180

200

220

240

260

280

Work (hp-hr)

All Cycles

N/V (rpm/(km/hr))

bsfc (g/hp-hr)

bsfc

450hp rating
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-90

Figure 3-19 Surface plot of CO2 with child rating engine for 55mph cycle

Figure 3-20 Surface plot of CO2 with child rating engine N/V for 65 mph cycle 

20

25

30

25 20 35 30 45 40

145

150

155

160

165

170

175

180

185

190

195

Work (hp-hr)

55mph

N/V (rpm/(km/hr))

bsfc (g/hp-hr)

bsfc

400hp rating

450hp rating

15

20

25

30

25 30 35 40 45 50

140

150

160

170

180

190

200

Work (hp-hr)

65mph

N/V (rpm/(km/hr))

bsfc (g/hp-hr)

bsfc

400hp rating

450hp rating
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-91

Figure 3-21 Surface plot of CO2 with child rating engine for ABB cycle

Figure 3-19 to Figure 3-21 demonstrate the well behaving nature of CO2 surface as a 

function of work and N/V as long as the surface fitting is conducted in an individual cycle, 

showing the potential to use surface fitting or interpolation scheme to determine the point 

resulted from the actual vehicle certification. However, please note that the entire simulations 

consist of over one thousand points, and it would be impossible to run all engine tests in order to 

generate a CO2 map for use in certification. Efforts must be made to greatly simplify the process 

by reducing the points to a minimum level, which can still very well represent a to-be-certified 

vehicle. After numerous trials, it is found that a minimum 9 points per cycle is needed, which 

covers three final drive ratios and three vehicle loads or work. Displayed in Figure 3-22 to 

Figure 3-24 are the same plots as Figure 3-19 to Figure 3-21, but the numbers of points are 

reduced to 9 points per cycle.

60

70

80

90

11.5 12 12.5 13 13.5 14 14.5

170

175

180

185

190

195

200

205

210

Work (hp-hr)

ARB

N/V (rpm/(km/hr))

bsfc (g/hp-hr)

bsfc

400hp rating

450hp rating
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-92

Figure 3-22 Contour plot of CO2 with only 9 points for 55mph cycle

Figure 3-23 Contour plot of CO2 with only 9 points N/V for 65 mph cycle 

20

25

30

20 25 30 35 40 45

140

150

160

170

180

190

200

Work (hp-hr)

55mph

N/V (rpm/(km/hr))

bsfc (g/hp-hr)

bsfc

All Points

9 Points

15

20

25

30

25 30

35 40

45 50

140

150

160

170

180

190

200

Work (hp-hr)

65mph

N/V (rpm/(km/hr))

bsfc (g/hp-hr)

bsfc

All Points

9 Points
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-93

Figure 3-24 Contour plot of CO2 with only 9 points for ABB cycle

Comparing Figure 3-19 to Figure 3-21, very similar behaviors for all three cycles are 

observed, and therefore, it can be said that 9 points would be acceptable at least for the engines

and vehicles that are under consideration. 

3.10.5 Generic Vehicle Definition 

Chapter 3.10.4 shows that nine vehicle configurations for each certification cycle could 

be used to cover the range of the vehicles to be certified. However, it is not clear how those 

vehicle configurations can be defined in a generic way. This section attempts to achieve this 

objective. 

To cover the range of vehicle configuration that the engine could be sold in the agencies 

are considering the vehicle configuration defined in Table 3-30 to Table 3-32. To cover the 

range of axles, the axle ratios would be calculated from the regulatory defined engines speeds 

and the maximum duty cycle speed of 65 mph. With the engine speed, tire radius, top gear ratio 

and vehicle speed, the axle ratio can be calculated. To cover the range of engine work the 

agencies are proposing a range of drag area (CdA), coefficients of rolling resistance (Crr) and 

vehicle masses. For Class 8 vehicles the highest mass and highest CdA be on the same vehicle 

and that both CdA and vehicle mass drop together for the lower average power vehicles. The 

reason for this is that, the cruise cycle’s average power is most affected by the vehicle’s CdA, 

where mass has the largest effect on average power on the transient cycle. For Class 2b-7 

60

70

80

90

11.5 12 12.5 13 13.5 14 14.5

170

175

180

185

190

195

200

205

210

Work (hp-hr)

ARB

N/V (rpm/(km/hr))

bsfc (g/hp-hr)

bsfc

All Points

9 Points
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-94

vehicles mass and Crr are varied to change the average power over the cycle. For these vehicles 

Crr is varied instead of CdA because CdA is not an input into GEM for vocational vehicles.

In addition to defining the vehicle parameters the agencies are proposing that the 

transmissions be defined for each vehicle configuration. Table 3-34 defines the transmission 

type and gear ratios for each of these vehicles.

Table 3-34 Default Transmissions

GEAR

NUMBER

TRANSMISSION TYPE AND GEAR RATIOS

10 speed AMT 6 speed HH AT 6 speed MH AT

1 12.8 4.6957 3.102

2 9.25 2.213 1.8107

3 6.76 1.5291 1.4063

4 4.9 1 1

5 3.58 0.7643 0.7117

6 2.61 0.6716 0.61

7 1.89 N/A

8 1.38

9 1

10 0.73

3.10.6 Certification Point Determination from Alternative

Table 3-30 to Table 3-32 define the numbers of vehicle configurations so that a welldefined map can be generated for a certain vehicle family to be certified. Section 3.10.4 shows 

that a surface fitting could approximate the surface for the entire vehicle applications. It 

indicates that the fitted surface could be well behaved only if an individual cycle is plotted. 

However, it can be imagined that the surface could be distorted if those mapping points shown in 

Table 3-30 to Table 3-32 may not be general enough to define the range of applications. As a 

result, an alternative to surface fitting should be considered. This section discusses a numerical 

scheme that is used to determine to-be-certified CO2 of the vehicle family from the CO2 map 

generated from the engine tests as well as GEM simulation. It should be pointed out that the 

numerical scheme discussed in this section is only the first attempt to derive a numerical scheme 

that can interpolate or extrapolate certification point, and many other alternatives and 

optimization schemes can be used as well.

In the approach, it is assumed that one map per certification cycle is considered, each 

consisting of 8-9 data points, but the map data does not need to be in an exact rectangular matrix 

or in any particular order. In order to demonstrate the concept, the generic vehicle defined in 

Table 3-31 is used, where there are a total of nine configurations as shown in Figure 3-25 to

Figure 3-27 for three certification cycles. Shown in these figures are also a number of vehicle 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-95

configurations that could be certified under this map, covering a wide range of applications 

including both tractor and vocational vehicles and different transmission packages. 

In these figures, X axis is SN/SV defined as average engine speed over average vehicle 

speed in rpm/km-ph. Y axis is the cycle work in kw-hr over the individual driving cycle. The 

legend of “Map Points” stands for the generic class 8 nine vehicle configurations using the same 

engine defined in Table 3-31. In these transmission packages, the same engine as the generic 

vehicle is used, and vehicle variables are varied, such as tire, axle, and aerodynamic packages. 

Veh 1 and Veh 2 stand for class 8 vehicles for typical vocational applications using the same 

engine as the generic vehicle. They all have similar vehicle weight, but with different tires, axle 

ratios, and aerodynamic packages. The purpose of these practices is to demonstrate whether the 

generic vehicle points (Map Points) can be used to interpolate or extrapolate all other points 

under different vehicle and transmission configurations with a high confidence level. 

From these figures, it can be seen that quite a few vehicle configurations are outside 

generic vehicle points, which are shown in “Map Points”. This means that extrapolations must 

be used. It is hoped that whatever numerical scheme is used can offer reasonably good accuracy

in terms of interpolation as well as extrapolation.

Figure 3-25 Alternative certification map for 55 mph certification cycle

16.0

17.0

18.0

19.0

20.0

21.0

22.0

23.0

24.0

25.0

26.0

27.0

28.0

29.0

30.0

10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0

kW-hr

SN/SV

gCO2/kW-hr

Map 

Points

Tran 1

Veh 1

Veh 2

Tran 2

Tran 3
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-96

Figure 3-26 Alternative certification map for 65 mph certification cycle

Figure 3-27 Alternative certification map for ARB certification cycle

19.0

20.0

21.0

22.0

23.0

24.0

25.0

26.0

27.0

28.0

29.0

30.0

31.0

32.0

33.0

34.0

10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0

kW-hr

SN/SV

gCO2/kW-hr

Map 

Points

Tran 1

Veh 1

Veh 2

Tran 2

Tran 3

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

37.5 38.5 39.5 40.5 41.5 42.5

kW-hr

SN/SV

gCO2/kW-hr

Map 

Points

Tran 1

Veh 1

Veh 2

Tran 2

Tran 3
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-97

In order to prove the concept and numerical accuracy in determining appropriate CO2 for 

certification, those nine points defined by Table 3-31 as shown in “Map Points” of these three 

figures which are used to interpolate or extrapolate CO2 for any other vehicle configurations 

points and then are compared with the actual simulation points. In the numerical scheme used in 

this study, within, above, and to the right of a mapped region defined by nine Map Points, 

interpolation and extrapolation occur in a plane defined by the interpolated point’s 3-nearest 

mapped neighbors. Below and to the left of the mapped region extrapolation occur between the 

2 nearest neighbors along the non-extrapolated axis and there is no change in the extrapolated 

value along any extrapolated axis. The interpolation scheme is to pick the 3 closest neighbors 

that have a maximum included angle less than 120 degrees, which eliminates picking the narrow 

triangular planes formed just outside of the mapped region. Table 3-35, Table 3-36, and Table 

3-37 show the comparisons for those points. 

In these tables, there are 76 vehicle configurations. Among them, nine points with green 

color filled are the generic vehicle defined in Table 3-31. All other points represent different 

vehicle configurations under the same engine family. The column with error (%) is the 

comparison of CO2 g/hp-hr between the test points and interpolated or extrapolated values. For 

the sake of simplicity, both values of CO2 g/hp-hr for both test and interpolated/extrapolate 

points are not shown in these tables. As can be seen from Table 3-35 and Table 3-36, only one 

point shows over 4 percent numerical accuracy, and most points are under 2.5 percent difference

for cruise speed 55 mph and 65 mph cycles, even for those points located outside mapped region. 

On the other hand, ARB cycle shows quite a few points in the range of 4.0-4.5 percent

difference, which is a concern as far as the certification is concerned. These results suggest that

more work and numerical scheme development will be required moving forward.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-98

Table 3-35 Proof of Concept and Numerical Scheme Accuracy for 55MPH Cycle

INTERP/EXTRAP POINTS INTERP/EXTRAP POINTS

i N/Vi kW-hri Error 

(%)

i N/Vi kW-hri Error 

(%)

1 12.61 27.37 0.5% 39 12.71 24.11 1.0%

2 12.31 27.73 -0.4% 40 14.18 24.17 0.4%

3 13.30 27.78 -0.2% 41 18.06 24.36 2.3%

4 12.47 23.81 1.6% 42 13.31 24.13 0.8%

5 12.31 24.09 1.0% 43 12.71 20.74 1.2%

6 13.30 24.13 0.8% 44 14.18 20.80 0.9%

7 12.42 20.46 1.8% 45 18.06 20.98 3.0%

8 12.31 20.72 1.7% 46 13.31 20.76 1.2%

9 13.30 20.76 1.2% 47 11.0727 20.474 0.5%

10 11.85 25.84 0.2% 48 11.3574 20.298 1.5%

11 12.80 25.89 0.3% 49 11.9048 20.342 1.9%

12 11.85 22.36 1.3% 50 12.8518 20.428 1.7%

13 12.80 22.40 1.6% 51 11.1814 22.769 0.3%

14 12.14 20.97 0.2% 52 11.3602 22.55 1.0%

15 12.04 21.19 -0.3% 53 11.9048 22.609 1.5%

16 13.01 21.24 -0.6% 54 12.8518 22.7 1.6%

17 14.19 21.30 -2.1% 55 12.882 26.145 0.5%

18 19.26 21.61 -2.0% 56 15.1703 21.851 -0.7%

19 11.99 20.14 0.3% 57 17.4585 22.132 -0.8%

20 12.04 20.33 -0.2% 58 19.7468 22.449 -1.8%

21 13.01 20.38 -0.9% 59 22.0351 22.745 -3.3%

22 14.19 20.44 -1.6% 60 23.1592 22.9 -4.0%

23 19.26 20.73 -1.4% 61 18.3492 22.253 -1.1%

24 11.98 19.36 0.4% 62 20.7542 22.579 -2.8%

25 12.04 19.54 0.0% 63 15.9442 21.943 -0.2%

26 13.01 19.59 -0.3% 64 13.5392 26.215 0.4%

27 14.19 19.65 -1.2% 65 10.8136 25.072 0.0%

28 19.26 19.93 -0.9% 66 10.4952 19.698 0.0%

29 14.44 21.32 -2.3% 67 10.4952 16.748 0.0%

30 14.90 21.34 -2.9% 68 14.4455 26.005 0.0%

31 12.97 21.24 -0.6% 69 14.4455 19.938 0.0%

32 17.08 21.47 -1.4% 70 14.4455 16.927 0.0%

33 13.82 21.28 -1.8% 71 19.5963 26.267 0.0%

34 18.55 21.56 -1.7% 72 19.5963 20.22 0.0%

35 12.71 27.75 -0.2% 73 19.5963 17.21 0.0%

36 14.18 27.82 -0.5% 74 17.0015 26.129 1.5%

37 18.06 28.03 -1.3% 75 17.0015 20.072 1.8%
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-99

38 13.31 27.78 -0.2% 76 17.0015 17.061 2.0%

Table 3-36 Proof of Concept and Numerical Scheme Accuracy for 65MPH Cycle

INTERP/EXTRAP POINTS INTERP/EXTRAP POINTS

i N/Vi kWhriError 

(%)

i N/Vi kW-hri Error 

(%)

1 11.40 32.68 0.6% 39 12.71 27.96 1.8%

2 12.31 32.85 -0.9% 40 14.18 28.04 0.8%

3 13.30 32.94 -1.2% 41 18.06 28.26 2.1%

4 11.40 27.86 0.9% 42 13.31 27.99 1.5%

5 12.31 27.93 1.8% 43 12.71 23.39 2.2%

6 13.30 27.99 1.5% 44 14.18 23.47 1.9%

7 11.40 23.33 1.5% 45 18.06 23.67 4.2%

8 12.31 23.38 2.1% 46 13.31 23.42 2.1%

9 13.30 23.42 2.1% 47 10.287 23.201 0.1%

10 11.85 30.33 1.1% 48 11.1382 23.307 1.3%

11 12.80 30.41 0.6% 49 11.9048 23.383 2.0%

12 11.85 25.57 1.9% 50 12.8518 23.471 2.3%

13 12.80 25.61 2.4% 51 10.4864 26.581 0.1%

14 11.15 27.02 1.1% 52 11.1382 26.689 0.9%

15 12.04 27.08 1.2% 53 11.9048 26.797 1.2%

16 13.01 27.13 1.7% 54 12.8518 26.895 2.3%

17 14.19 27.21 0.4% 55 12.882 31.126 0.1%

18 19.26 27.57 1.2% 56 15.1703 27.695 0.5%

19 11.15 25.75 1.2% 57 17.4585 28.007 1.6%

20 12.04 25.80 2.0% 58 19.7468 28.315 0.1%

21 13.01 25.86 0.7% 59 22.0351 25.395 2.3%

22 14.19 25.94 0.2% 60 23.1592 23.919 3.4%

23 19.26 26.30 1.0% 61 18.3492 28.124 2.0%

24 11.15 24.56 0.3% 62 20.7542 27.401 -0.3%

25 12.04 24.61 1.5% 63 15.9442 27.802 1.0%

26 13.01 24.67 0.4% 64 13.5392 31.201 -0.3%

27 14.19 24.75 -0.3% 65 10.4952 29.49 0.0%

28 19.26 25.11 0.4% 66 10.4953 23.234 0.0%

29 14.44 27.23 0.0% 67 10.4952 19.715 0.0%

30 14.90 27.26 -0.7% 68 14.4455 29.917 0.0%

31 12.97 27.13 1.8% 69 14.4456 23.441 0.0%

32 17.08 27.41 1.5% 70 14.4455 19.921 0.0%

33 13.82 27.18 0.8% 71 19.5963 30.142 0.0%

34 18.55 27.52 2.0% 72 19.5964 23.766 0.0%

35 12.71 32.89 -1.0% 73 19.5963 20.247 0.0%
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-100

36 14.18 33.00 -1.8% 74 17.0015 30.063 1.0%

37 18.06 33.20 -1.6% 75 17.0016 23.594 0.6%

38 13.31 32.94 -1.2% 76 17.0015 20.075 0.3%

Table 3-37 Proof Of Concept and Numerical Scheme Accuracy for ARB Cycle

INTERP/EXTRAP POINTS INTERP/EXTRAP POINTS

i N/Vi kWhriError 

(%)

i N/Vi kW-hri Error 

(%)

1 41.00 9.66 -1.2% 39 39.26 9.29 -2.7%

2 40.16 9.65 -0.9% 40 40.34 9.30 -1.3%

3 39.48 9.64 -1.0% 41 40.56 9.33 -1.9%

4 40.98 9.30 -1.0% 42 39.49 9.28 -2.5%

5 40.13 9.30 -2.9% 43 39.23 8.92 -2.3%

6 39.43 9.28 -2.3% 44 40.31 8.93 -2.6%

7 40.95 8.93 -2.1% 45 40.53 8.97 -2.8%

8 40.03 8.93 -2.0% 46 39.00 8.91 -2.0%

9 39.39 8.91 -1.9% 47 39.1403 9.2227 1.0%

10 41.40 9.48 -0.7% 48 40.0123 9.2143 0.4%

11 39.47 9.45 -1.3% 49 39.2724 9.1879 0.7%

12 41.13 9.10 -1.1% 50 38.9924 9.1411 1.0%

13 39.43 9.09 -2.3% 51 39.2179 9.3688 0.4%

14 40.17 6.25 -4.1% 52 40.0236 9.3563 0.2%

15 39.40 6.25 -4.2% 53 39.3053 9.3359 0.4%

16 38.82 6.26 -4.3% 54 38.9921 9.2828 0.9%

17 38.89 6.26 -4.4% 55 39.1243 10.353 1.2%

18 40.73 6.32 -3.9% 56 38.1574 7.0408 0.2%

19 40.03 6.17 -4.5% 57 40.5059 7.0397 0.7%

20 39.40 6.17 -4.5% 58 42.0372 7.087 1.2%

21 38.75 6.19 -4.6% 59 43.4797 7.1226 1.6%

22 38.88 6.18 -4.6% 60 43.1689 7.1632 0.9%

23 40.73 6.25 -4.1% 61 44.334 7.1471 0.8%

24 40.02 6.11 -4.8% 62 44.0081 7.1626 0.0%

25 39.31 6.11 -4.7% 63 41.7231 7.0508 -2.9%

26 38.75 6.12 -4.8% 64 41.8206 10.136 1.5%

27 38.88 6.12 -4.8% 65 39.4411 11.832 0.0%

28 40.73 6.18 -4.4% 66 38.2432 8.6276 0.0%

29 39.85 6.27 -4.2% 67 37.8895 7.3752 0.0%

30 40.32 6.29 -3.9% 68 41.2466 11.14 0.0%

31 38.40 6.23 -4.5% 69 40.3708 8.1927 0.0%

32 40.88 6.73 -3.3% 70 40.0346 7.1138 0.0%

33 38.21 6.34 -4.5% 71 42.1071 10.8 0.0%
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-101

34 41.58 6.69 -2.9% 72 41.6921 8.0995 0.0%

35 39.31 9.65 -1.3% 73 41.4929 7.0552 0.0%

36 40.36 9.66 -1.3% 74 42.1548 10.83 0.2%

37 40.69 9.69 -1.8% 75 41.6017 8.0873 0.4%

38 39.57 9.64 -1.2% 76 41.4366 7.0472 0.4%

3.10.7 Preliminary Comparisons of Primary and Alternative Approaches

The main purpose of introduction of this alternative approach for vehicle certification is 

to integrate vehicle certification with engine certification, being able to address both GHG 

emissions and criteria emission in one set of tests. Chapter 3.10.4 shows the principle of this 

approach. However, one of the key questions remains – how this alternative certification

approach is credible in terms of accuracy as opposed to the primary vehicle certification 

approach? In addition, there are also many other questions that still need answers, such as parent 

and child rating impacts, the impact of vocational sector with a large variation on the ratio of 

N/V (average engine speed over average vehicle speed), surface fit or interpolation scheme, and 

engine certification. 

Shown in Figure 3-28 is the comparison between alternative approaches discussed in this 

Section and GEM with steady state map based approach discussed in Chapter 4 of this draft RIA. 

In both engine and GEM simulations, a test matrix consisting of twenty seven points is tested (3 

axle ratios x 3 Cd values x 3 cycles) as shown below.

• Nine variations of the Kenworth T700 to cover the range of vehicle loads

– Axel ratio: 2.64, 2.85, and 3.08

– Cd: 0.41, 0.55, and 0.7

• Three certification cycles – with distance correction 

– 55 w/SwRI grade profile

– 65mph w/SwRI grade profile

– ARB Transient

• Eaton AMT model: 10 speed: F016E310C-LAS
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-102

Figure 3-28 Comparisons between Alternative and GEM Based Approaches

120

130

140

150

160

170

180

190

130 140 150 160 170 180 190

Engine BSFC (g/hp-hr) - GEM

Engine BSFC (g/hp-hr) - measured from Engine tests

1:1

GEM sim - SS map based

Alternative

1:1 +5%

1:1 -5%
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-103

As shown in Figure 3-28, both alternative approach and GEM based approach seem to 

produce similar results in 55 mph and 65 mph cruise cycles with road grade. However, it is 

clearly shown that the alternative approach seems to be better in the transient cycle than GEM 

based approach with the steady state map.

3.10.8 Remaining Questions and Future Plans

The comparison shown in Figure 3-28 only partially answers this alternative approach 

fidelity with one specific engine and transmission platform and because of this there are still 

many unanswered questions. The following questions are only a subset of the questions that 

need to be answered:

 How does this alternative approach address parent and child rating? Would nine 

points be adequate to cover the practical range of the vehicle operation on the road?

 How can this approach address those points that may be out of map ranges with much 

higher or lower N/V? 

 What kinds of numerical schemes, interpolation or surface fitting, shall be used to 

interpolate those points that are located between testing points?

 What the numerical scheme shall be used to extrapolate those points outside the 

maps?

 How robust are these numerical schemes?

 Can what we have learned so far be applied to other engines?

 How the single engine certification point shall be selected among the number of 

engine tests? 

 Are there potential unintended consequences?

The agencies welcome comments on these questions and on the alternative vehicle certification 

approach in general. 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-104

References

1

For more information, see CFR Title 40, Part 86.004-28

2 National Renewable Energy Laboratory, 2014. Technical Report: Appending High-Resolution Elevation Data to 

GPS Speed Traces for Vehicle Energy Modeling and Simulation. http://www.nrel.gov/docs/fy14osti/61109.pdf

3

Southwest Research Institute coastdown and constant speed testing summary report for EPA.

4 R.M. Young. Product specifications for Ultrasonic Anemometer Model 81000. 

http://www.youngusa.com/products/6/3.html

5

SAE International, 2013, Guidelines for Aerodynamic Assessment of Medium and Heavy Commercial Ground 

Vehicles Using Computational Fluid Dynamics, SAE J2966, 2013-09

6

2010 NAS Report. Finding 2-4 on page 39.

7

SAE International, 1987, Fuel Consumption In-Service Test Procedure Type III, SAE J1526, 1987-06 

8 CARB-NREL Aero testing Report (2014)

9 U.S. EPA. Truck and Trailer Roof Height Match Analysis Memorandum from Amy Kopin to the Docket, August 

9, 2010. Docket Identification Number EPA-HQ-OAR-2010-0162-0045. 

10 Ben Sharpe (ICCT) and Mike Roeth (North American Council for Freight Efficiency), “Costs and Adoption Rates 

of Fuel-Saving Technologies for Trailer in the North American On-Road Freight Sector”, Feb 2014.

11 Frost & Sullivan, “Strategic Analysis of North American Semi-trailer Advanced Technology Market”, Feb 2013.

12 EPA, 2011, HD Phase 1 Regulatory Impact Analysis, Figure 3-16. EPA-420-R-11-901.

13 (http://webstore.ansi.org/RecordDetail.aspx?sku=ISO+28580%3a2009)

14 SAE International, 2006, Rolling Resistance measurement Procedure for Passenger Car, Light Truck, and 

Highway Truck and Bus Tires, SAE J1269, 2006-09

15 ISO, 2009, Passenger Car, Truck, and Bus Tyres – Methods of Measuring Rolling Resistance – Single Point Test 

and Correlation of Measurement Results: ISO 28580:2009(E), First Edition, 2009-07-01

16 NHTSA, 2009. “NHTSA Tire Fuel Efficiency Consumer Information Program Development: Phase 1 –

Evaluation of Laboratory Test Protocols.” DOT HS 811 119. June. (www.regulations.gov, Docket ID: NHTSA2008-0121-0019).

17 SAE International, 1999, Stepwise Coastdown Methodology for Measuring Tire Rolling Resistance, SAE J2452, 

1999-06

18 ISO, 2005, Passenger Car, Truck, Bus, and Motorcycle Tyres – Methods of Measuring Rolling Resistance, ISO 

18164:2005(E)

19 SAE International, 2012, Test Procedures for Measuring Truck Tire Revolutions Per Kilometer/Mile, SAE J1025, 

2012-08

20 Based on MOVES analysis.

21

 Gautam, M., N. Clark, W. Riddle, R. Nine, S. Wayne, H. Maldonado, A. Agrawal, M. Carlock. “Development 

and Initial Use of a Heavy-Duty Diesel Truck Test Schedule for Emissions Characterization.” SAE Paper 2002-01-

1753. 2002.

22 Governors Highway Safety Association. Speed Limit Laws May 2011. Last viewed on May 9, 2011 at 

http://www.ghsa.org/html/stateinfo/laws/speedlimit_laws.html

23 Environmental Defense Fund. “Greenhouse Gas Management for Medium-Duty Truck Fleets.” Viewed at 

http://edf.org/documents/10860_fleets-med-ghg-management.pdf. Page 6.

24 ICF International. Investigation of Costs for Strategies to Reduce Greenhouse Gas Emissions for Heavy-Duty 

On-Road Vehicles. July 2010. Pages 4-16. Docket Identification Number EPA-HQ-OAR-2010-0162-0044.

25 M.J. Bradley & Associates. Setting the Stage for Regulation of Heavy-Duty Vehicle Fuel Economy and GHG 

Emissions: Issues and Opportunities. February 2009. Page 35. Analysis based on 1992 Truck Inventory and Use 

Survey data, where the survey data allowed developing the distribution of loads instead of merely the average loads.

26 The U.S. Federal Highway Administration. Development of Truck Payload Equivalent Factor. Table 11. Last 

viewed on March 9, 2010 at 

http://ops.fhwa.dot.gov/freight/freight_analysis/faf/faf2_reports/reports9/s510_11_12_tables.htm
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

3-105

27 Excerpted from The U.S. Federal Highway Administration. Development of Truck Payload Equivalent Factor. 

Table 11. Last viewed on March 9, 2010 at 

http://ops.fhwa.dot.gov/freight/freight_analysis/faf/faf2_reports/reports9/s510_11_12_tables.htm

28 The U.S. Federal Highway Administration. Development of Truck Payload Equivalent Factor. Table 11. Last 

viewed on March 9, 2010 at 

http://ops.fhwa.dot.gov/freight/freight_analysis/faf/faf2_reports/reports9/s510_11_12_tables.htm